Next Greater Element

# Next Greater Element Notes - Class 8

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Given an array, print the Next Greater Element (NGE) for every element. The Next greater Element for an element x is the first greater element on the right side of x in the array. Elements for which no greater element exist, consider the next greater element as -1.
Examples:

• For an array, the rightmost element always has the next greater element as -1.
• For an array that is sorted in decreasing order, all elements have the next greater element as -1.
• For the input array [4, 5, 2, 25], the next greater elements for each element are as follows. For the input array [13, 7, 6, 12}, the next greater elements for each element are as follows. Method 1 (Simple)

Use two loops: The outer loop picks all the elements one by one. The inner loop looks for the first greater element for the element picked by the outer loop. If a greater element is found then that element is printed as next, otherwise, -1 is printed.
Below is the implementation of the above approach:

C++

// Simple C++ program to print

// next greater elements in a

// given array

#include<iostream>

using namespace std;

/* prints element and NGE pair

for all elements of arr[] of size n */

void printNGE(int arr[], int n)

{

int next, i, j;

for (i = 0; i < n; i++)

{

next = -1;

for (j = i + 1; j < n; j++)

{

if (arr[i] < arr[j])

{

next = arr[j];

break;

}

}

cout << arr[i] << " -- "

<< next << endl;

}

}

// Driver Code

int main()

{

int arr[] = {11, 13, 21, 3};

int n = sizeof(arr)/sizeof(arr);

printNGE(arr, n);

return 0;

}

// This code is contributed

// by Akanksha Rai(Abby_akku)

C

// Simple C program to print next greater elements

// in a given array

#include<stdio.h>

/* prints element and NGE pair for all elements of

arr[] of size n */

void printNGE(int arr[], int n)

{

int next, i, j;

for (i=0; i<n; i++)

{

next = -1;

for (j = i+1; j<n; j++)

{

if (arr[i] < arr[j])

{

next = arr[j];

break;

}

}

printf("%d -- %dn", arr[i], next);

}

}

int main()

{

int arr[]= {11, 13, 21, 3};

int n = sizeof(arr)/sizeof(arr);

printNGE(arr, n);

return 0;

}

Java

// Simple Java program to print next

// greater elements in a given array

class Main

{

/* prints element and NGE pair for

all elements of arr[] of size n */

static void printNGE(int arr[], int n)

{

int next, i, j;

for (i=0; i<n; i++)

{

next = -1;

for (j = i+1; j<n; j++)

{

if (arr[i] < arr[j])

{

next = arr[j];

break;

}

}

System.out.println(arr[i]+" -- "+next);

}

}

public static void main(String args[])

{

int arr[]= {11, 13, 21, 3};

int n = arr.length;

printNGE(arr, n);

}

}

Python

# Function to print element and NGE pair for all elements of list

def printNGE(arr):

for i in range(0, len(arr), 1):

next = -1

for j in range(i+1, len(arr), 1):

if arr[i] < arr[j]:

next = arr[j]

break

print(str(arr[i]) + " -- " + str(next))

# Driver program to test above function

arr = [11,13,21,3]

printNGE(arr)

# This code is contributed by Sunny Karira

C#

// Simple C# program to print next

// greater elements in a given array

using System;

class GFG

{

/* prints element and NGE pair for

all elements of arr[] of size n */

static void printNGE(int []arr, int n)

{

int next, i, j;

for (i = 0; i < n; i++)

{

next = -1;

for (j = i + 1; j < n; j++)

{

if (arr[i] < arr[j])

{

next = arr[j];

break;

}

}

Console.WriteLine(arr[i] + " -- " + next);

}

}

// driver code

public static void Main()

{

int []arr= {11, 13, 21, 3};

int n = arr.Length;

printNGE(arr, n);

}

}

// This code is contributed by Sam007

PHP

<?php

// Simple PHP program to print next

// greater elements in a given array

/* prints element and NGE pair for

all elements of arr[] of size n */

function printNGE(\$arr, \$n)

{

for (\$i = 0; \$i < \$n; \$i++)

{

\$next = -1;

for (\$j = \$i + 1; \$j < \$n; \$j++)

{

if (\$arr[\$i] < \$arr[\$j])

{

\$next = \$arr[\$j];

break;

}

}

echo \$arr[\$i]." -- ". \$next."\n";

}

}

// Driver Code

\$arr= array(11, 13, 21, 3);

\$n = count(\$arr);

printNGE(\$arr, \$n);

// This code is contributed by Sam007

?>

JavaScript

<script>

// Simple JavaScript program to print

// next greater elements in a

// given array

/* prints element and NGE pair

for all elements of arr[] of size n */

function printNGE(arr, n)

{

var next, i, j;

for (i = 0; i < n; i++)

{

next = -1;

for (j = i + 1; j < n; j++)

{

if (arr[i] < arr[j])

{

next = arr[j];

break;

}

}

document.write(arr[i] + " -- " + next);

document.write("<br>");

}

}

// Driver Code

var arr = [11, 13, 21, 3];

var n = arr.length;

printNGE(arr, n);

// This code is contributed by rdtank.

</script>

Output

11 -- 13

13 -- 21

21 -- -1

3 -- -1

Time Complexity: O(N2)
Auxiliary Space: O(1)

Method 2 (Using Stack)

• Push the first element to stack.
• Pick rest of the elements one by one and follow the following steps in loop.
• Mark the current element as next.
• If stack is not empty, compare top element of stack with next.
• If next is greater than the top element, Pop element from stack. next is the next greater element for the popped element.
• Keep popping from the stack while the popped element is smaller than next. next becomes the next greater element for all such popped elements.
• Finally, push the next in the stack.
• After the loop in step 2 is over, pop all the elements from the stack and print -1 as the next element for them.

Below image is a dry run of the above approach: Below is the implementation of the above approach:

C++

// A Stack based C++ program to find next

// greater element for all array elements.

#include <bits/stdc++.h>

using namespace std;

/* prints element and NGE pair for all

elements of arr[] of size n */

void printNGE(int arr[], int n)

{

stack<int> s;

/* push the first element to stack */

s.push(arr);

// iterate for rest of the elements

for (int i = 1; i < n; i++)

{

if (s.empty()) {

s.push(arr[i]);

continue;

}

/* if stack is not empty, then

pop an element from stack.

If the popped element is smaller

than next, then

a) print the pair

b) keep popping while elements are

smaller and stack is not empty */

while (s.empty() == false

&& s.top() < arr[i])

{

cout << s.top()

<< " --> " << arr[i] << endl;

s.pop();

}

/* push next to stack so that we can find

next greater for it */

s.push(arr[i]);

}

/* After iterating over the loop, the remaining

elements in stack do not have the next greater

element, so print -1 for them */

while (s.empty() == false) {

cout << s.top() << " --> " << -1 << endl;

s.pop();

}

}

/* Driver code */

int main()

{

int arr[] = { 11, 13, 21, 3 };

int n = sizeof(arr) / sizeof(arr);

printNGE(arr, n);

return 0;

}

C

// A Stack based C program to find next

//  greater element for all array elements.

#include <stdbool.h>

#include <stdio.h>

#include <stdlib.h>

#define STACKSIZE 100

// stack structure

struct stack {

int top;

int items[STACKSIZE];

};

// Stack Functions to be used by printNGE()

void push(struct stack* ps, int x)

{

if (ps->top == STACKSIZE - 1) {

printf("Error: stack overflown");

getchar();

exit(0);

}

else {

ps->top += 1;

int top = ps->top;

ps->items[top] = x;

}

}

bool isEmpty(struct stack* ps)

{

return (ps->top == -1) ? true : false;

}

int pop(struct stack* ps)

{

int temp;

if (ps->top == -1) {

printf("Error: stack underflow n");

getchar();

exit(0);

}

else {

int top = ps->top;

temp = ps->items[top];

ps->top -= 1;

return temp;

}

}

/* prints element and NGE pair for all elements of

arr[] of size n */

void printNGE(int arr[], int n)

{

int i = 0;

struct stack s;

s.top = -1;

int element, next;

/* push the first element to stack */

push(&s, arr);

// iterate for rest of the elements

for (i = 1; i < n; i++) {

next = arr[i];

if (isEmpty(&s) == false)

{

// if stack is not empty, then pop an element

// from stack

element = pop(&s);

/* If the popped element is smaller than next,

then a) print the pair b) keep popping while

elements are smaller and stack is not empty

*/

while (element < next) {

printf("n %d --> %d", element, next);

if (isEmpty(&s) == true)

break;

element = pop(&s);

}

/* If element is greater than next, then push

the element back */

if (element > next)

push(&s, element);

}

/* push next to stack so that we can find

next greater for it */

push(&s, next);

}

/* After iterating over the loop, the remaining

elements in stack do not have the next greater

element, so print -1 for them */

while (isEmpty(&s) == false)

{

element = pop(&s);

next = -1;

printf("n %d --> %d", element, next);

}

}

/* Driver code */

int main()

{

int arr[] = { 11, 13, 21, 3 };

int n = sizeof(arr) / sizeof(arr);

printNGE(arr, n);

getchar();

return 0;

}

Java

// Java program to print next

// greater element using stack

public class NGE {

static class stack {

int top;

int items[] = new int;

// Stack functions to be used by printNGE

void push(int x)

{

if (top == 99)

{

System.out.println("Stack full");

}

else

{

items[++top] = x;

}

}

int pop()

{

if (top == -1)

{

System.out.println("Underflow error");

return -1;

}

else {

int element = items[top];

top--;

return element;

}

}

boolean isEmpty()

{

return (top == -1) ? true : false;

}

}

/* prints element and NGE pair for

all elements of arr[] of size n */

static void printNGE(int arr[], int n)

{

int i = 0;

stack s = new stack();

s.top = -1;

int element, next;

/* push the first element to stack */

s.push(arr);

// iterate for rest of the elements

for (i = 1; i < n; i++)

{

next = arr[i];

if (s.isEmpty() == false)

{

// if stack is not empty, then

// pop an element from stack

element = s.pop();

/* If the popped element is smaller than

next, then a) print the pair b) keep

popping while elements are smaller and

stack is not empty */

while (element < next)

{

System.out.println(element + " --> "

+ next);

if (s.isEmpty() == true)

break;

element = s.pop();

}

/* If element is greater than next, then

push the element back */

if (element > next)

s.push(element);

}

/* push next to stack so that we can find next

greater for it */

s.push(next);

}

/* After iterating over the loop, the remaining

elements in stack do not have the next greater

element, so print -1 for them */

while (s.isEmpty() == false)

{

element = s.pop();

next = -1;

System.out.println(element + " -- " + next);

}

}

// Driver Code

public static void main(String[] args)

{

int arr[] = { 11, 13, 21, 3 };

int n = arr.length;

printNGE(arr, n);

}

}

// Thanks to Rishabh Mahrsee for contributing this code

Python

# Python program to print next greater element using stack

# Stack Functions to be used by printNGE()

def createStack():

stack = []

return stack

def isEmpty(stack):

return len(stack) == 0

def push(stack, x):

stack.append(x)

def pop(stack):

if isEmpty(stack):

print("Error : stack underflow")

else:

return stack.pop()

'''prints element and NGE pair for all elements of

arr[] '''

def printNGE(arr):

s = createStack()

element = 0

next = 0

# push the first element to stack

push(s, arr)

# iterate for rest of the elements

for i in range(1, len(arr), 1):

next = arr[i]

if isEmpty(s) == False:

# if stack is not empty, then pop an element from stack

element = pop(s)

'''If the popped element is smaller than next, then

a) print the pair

b) keep popping while elements are smaller and

stack is not empty '''

while element < next:

print(str(element) + " -- " + str(next))

if isEmpty(s) == True:

break

element = pop(s)

'''If element is greater than next, then push

the element back '''

if element > next:

push(s, element)

'''push next to stack so that we can find

next greater for it '''

push(s, next)

'''After iterating over the loop, the remaining

elements in stack do not have the next greater

element, so print -1 for them '''

while isEmpty(s) == False:

element = pop(s)

next = -1

print(str(element) + " -- " + str(next))

# Driver code

arr = [11, 13, 21, 3]

printNGE(arr)

# This code is contributed by Sunny Karira

C#

using System;

// c# program to print next

// greater element using stack

public class NGE {

public class stack {

public int top;

public int[] items = new int;

// Stack functions to be used by printNGE

public virtual void push(int x)

{

if (top == 99) {

Console.WriteLine("Stack full");

}

else {

items[++top] = x;

}

}

public virtual int pop()

{

if (top == -1) {

Console.WriteLine("Underflow error");

return -1;

}

else {

int element = items[top];

top--;

return element;

}

}

public virtual bool Empty

{

get { return (top == -1) ? true : false; }

}

}

/* prints element and NGE pair for

all elements of arr[] of size n */

public static void printNGE(int[] arr, int n)

{

int i = 0;

stack s = new stack();

s.top = -1;

int element, next;

/* push the first element to stack */

s.push(arr);

// iterate for rest of the elements

for (i = 1; i < n; i++) {

next = arr[i];

if (s.Empty == false) {

// if stack is not empty, then

// pop an element from stack

element = s.pop();

/* If the popped element is smaller than

next, then a) print the pair b) keep

popping while elements are smaller and

stack is not empty */

while (element < next) {

Console.WriteLine(element + " --> "

+ next);

if (s.Empty == true) {

break;

}

element = s.pop();

}

/* If element is greater than next, then

push the element back */

if (element > next) {

s.push(element);

}

}

/* push next to stack so that we can find next

greater for it */

s.push(next);

}

/* After iterating over the loop, the remaining

elements in stack do not have the next greater

element, so print -1 for them */

while (s.Empty == false) {

element = s.pop();

next = -1;

Console.WriteLine(element + " -- " + next);

}

}

// Driver Code

public static void Main(string[] args)

{

int[] arr = new int[] { 11, 13, 21, 3 };

int n = arr.Length;

printNGE(arr, n);

}

}

// This code is contributed by Shrikant13

Javascript

<script>

// A Stack based Javascript program to find next

// greater element for all array elements.

/* prints element and NGE pair for all

elements of arr[] of size n */

function printNGE(arr, n)

{

var s = [];

/* push the first element to stack */

s.push(arr);

// iterate for rest of the elements

for (var i = 1; i < n; i++)

{

if (s.length == 0) {

s.push(arr[i]);

continue;

}

/* if stack is not empty, then

pop an element from stack.

If the popped element is smaller

than next, then

a) print the pair

b) keep popping while elements are

smaller and stack is not empty */

while (s.length ==0 == false

&& s[s.length-1] < arr[i])

{

document.write( s[s.length-1]

+ " --> " + arr[i]+"<br>");

s.pop();

}

/* push next to stack so that we can find

next greater for it */

s.push(arr[i]);

}

/* After iterating over the loop, the remaining

elements in stack do not have the next greater

element, so print -1 for them */

while (s.length !=0) {

document.write( s[s.length-1] + " --> " + -1+ "<br>" );

s.pop();

}

}

/* Driver code */

var arr = [11, 13, 21, 3];

var n = arr.length;

printNGE(arr, n);

</script>

Output

11 --> 13

13 --> 21

3 --> -1

21 --> -1

Time Complexity: O(N)

Auxiliary Space: O(N)

The worst case occurs when all elements are sorted in decreasing order. If elements are sorted in decreasing order, then every element is processed at most 4 times.

• Initially pushed to the stack.
• Popped from the stack when next element is being processed.
• Pushed back to the stack because the next element is smaller.
• Popped from the stack in step 3 of the algorithm.

How to get elements in the same order as input?
The above approach may not produce output elements in the same order as the input. To achieve the same order, we can traverse the same in reverse order
Below is the implementation of the above approach:

C++

// A Stack based C++ program to find next

// greater element for all array elements

// in same order as input.

#include <bits/stdc++.h>

using namespace std;

/* prints element and res pair for all

elements of arr[] of size n */

void printNGE(int arr[], int n)

{

stack<int> s;

int res[n];

for (int i = n - 1; i >= 0; i--) {

/* if stack is not empty, then

pop an element from stack.

If the popped element is smaller

than next, then

a) print the pair

b) keep popping while elements are

smaller and stack is not empty */

if (!s.empty()) {

while (!s.empty() && s.top() <= arr[i]) {

s.pop();

}

}

res[i] = s.empty() ? -1 : s.top();

s.push(arr[i]);

}

for (int i = 0; i < n; i++)

cout << arr[i] << " --> " << res[i] << endl;

}

// Driver Code

int main()

{

int arr[] = { 11, 13, 21, 3 };

int n = sizeof(arr) / sizeof(arr);

// Function call

printNGE(arr, n);

return 0;

}

Java

// A Stack based Java program to find next

// greater element for all array elements

// in same order as input.

import java.util.Stack;

class NextGreaterElement {

static int arr[] = { 11, 13, 21, 3 };

/* prints element and NGE pair for all

elements of arr[] of size n */

public static void printNGE()

{

Stack<Integer> s = new Stack<>();

int nge[] = new int[arr.length];

// iterate for rest of the elements

for (int i = arr.length - 1; i >= 0; i--)

{

/* if stack is not empty, then

pop an element from stack.

If the popped element is smaller

than next, then

a) print the pair

b) keep popping while elements are

smaller and stack is not empty */

if (!s.empty())

{

while (!s.empty()

&& s.peek() <= arr[i])

{

s.pop();

}

}

nge[i] = s.empty() ? -1 : s.peek();

s.push(arr[i]);

}

for (int i = 0; i < arr.length; i++)

System.out.println(arr[i] +

" --> " + nge[i]);

}

/* Driver Code */

public static void main(String[] args)

{

// NextGreaterElement nge = new

// NextGreaterElement();

printNGE();

}

}

Python3

# Python code to implement the approach

# prints element and NGE pair for all

# elements of arr[] of size n

def printNGE(arr, n):

s = []

res = [0 for i in range(n)]

# iterate for rest of the elements

for i in range(n-1,-1,-1):

# /* if stack is not empty, then

# pop an element from stack.

# If the popped element is smaller

# than next, then

# a) print the pair

# b) keep popping while elements are

# smaller and stack is not empty */

if (len(s) != 0):

while (len(s) != 0 and s[-1] <= arr[i]):

s.pop()

res[i] = -1 if len(s) == 0 else s[-1]

s.append(arr[i])

for i in range(len(arr)):

print(str(arr[i]) + " --> " + str(res[i]))

# Driver Code

arr = [ 11, 13, 21, 3 ]

n = len(arr)

# Function call

printNGE(arr, n)

# This code is contributed by shinjanpatra

C#

// A Stack based C# program to find next

// greater element for all array elements

// in same order as input.

using System;

using System.Collections.Generic;

class GFG {

private int[] arr = new int[] { 11, 13, 21, 3 };

/* prints element and NGE pair for all

elements of arr[] of size n */

private void printNGE()

{

Stack<int> s = new Stack<int>();

int[] nge = new int[arr.Length];

// iterate for rest of the elements

for (int i = arr.Length - 1; i >= 0; i--)

{

/* if stack is not empty, then

pop an element from stack.

If the popped element is smaller

than next, then

a) print the pair

b) keep popping while elements are

smaller and stack is not empty */

if (s.Count > 0)

{

while (s.Count > 0

&& s.Peek() <= arr[i])

{

s.Pop();

}

}

nge[i] = s.Count == 0 ? -1 : s.Peek();

s.Push(arr[i]);

}

for (int i = 0; i < arr.Length; i++)

{

Console.WriteLine(arr[i] + " --> " + nge[i]);

}

}

// Driver Code

public static void Main(string[] args)

{

GFG nge = new GFG();

nge.printNGE();

}

}

Javascript

<script>

// Javascript program for the above approach

/* prints element and NGE pair for all

elements of arr[] of size n */

function printNGE(arr, n)

{

var s = [];

let res = new Array(n);

// iterate for rest of the elements

for (let i = n - 1; i >= 0; i--)

{

/* if stack is not empty, then

pop an element from stack.

If the popped element is smaller

than next, then

a) print the pair

b) keep popping while elements are

smaller and stack is not empty */

if (s.length != 0)

{

while (s.length != 0

&& s[s.length-1] <= arr[i])

{

s.pop();

}

}

res[i] = s.length == 0 ? -1 : s[s.length-1];

s.push(arr[i]);

}

for (let i = 0; i < arr.length; i++)

document.write(arr[i] +

" --> " + res[i] + "<br/>");

}

// Driver Code

let arr = [ 11, 13, 21, 3 ];

let n = arr.length;

// Function call

prletNGE(arr, n);

// This code is contributed by splevel62.

</script>

Output

11 ---> 13

13 ---> 21

21 ---> -1

3 ---> -1

Time Complexity: O(N)

Auxiliary Space: O(N)

Method 3:

• This is same as above method but the elements are pushed and popped only once into the stack. The array is changed in place. The array elements are pushed into the stack until it finds a greatest element in the right of array. In other words the elements are popped from stack when top of the stack value is smaller in the current array element.
• Once all the elements are processed in the array but stack is not empty. The left out elements in the stack doesn’t encounter any greatest element . So pop the element from stack and change it’s index value as -1 in the array.

Python3

# Python3 code

class Solution:

def nextLargerElement(self,arr,n):

#code here

s=[]

for i in range(len(arr)):

while s and s[-1].get("value") < arr[i]:

d = s.pop()

arr[d.get("ind")] = arr[i]

s.append({"value": arr[i], "ind": i})

while s:

d = s.pop()

arr[d.get("ind")] = -1

return arr

if __name__ == "__main__":

print(Solution().nextLargerElement([6,8,0,1,3],5))

Output

[8, -1, 1, 3, -1]

Time Complexity: O(N)

Auxiliary Space: O(N)

The document Next Greater Element Notes - Class 8 is a part of Class 8 category.
All you need of Class 8 at this link: Class 8
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