Next Greater Element

Posted: 16 Oct, 2020
Difficulty: Moderate

PROBLEM STATEMENT

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You are given an array arr of length N. You have to return a list of integers containing the NGE(next greater element) of each element of the given array. The NGE for an element X is the first greater element on the right side of X in the array. Elements for which no greater element exists, consider the NGE as -1.

For Example :

If the given array is [1, 3, 2], then you need to return [3, -1, -1]. Because for 1, 3 is the next greater element, for 3 it does not have any greater number to its right, and similarly for 2.
Input Format :
The first line of input contains a single integer T, representing the number of test cases or queries to be run. 

Then the T test cases follow.

The first line of each test case contains an integer N, representing the length of the input array(ARR).

The second line contains N single space-separated integers representing elements of the array arr.
Output Format :
For each test case, print a list of integers each representing the NGE(next greater element) of each element of the given array in a single line.
Note :
You are not required to print the expected output, it has already been taken care of. Just implement the function.
Constraints :
1 <= T <= 10
1 <= N <= 10^5
0 <= ARR[i] <= 10^9

Time Limit: 1sec
Approach 1
  1. Initialise an array ans of length N to store the next greater number of arr[i] at index i.
  2. Traverse the given array and for every element at index I
    • Initialise a variable next to -1.
    • Run a loop from i+1 to N, and check whether the current element is greater than arr[i] then update the next to the current element and break the loop.
    • Store next at ans[i].
  3. Return ans array.
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