Print next greater number of Q queries

Given an array of n elements and q queries, for each query that has index i, find the next greater element and print its value. If there is no such greater element to its right then print -1. 
Examples:

Input : arr[] = {3, 4, 2, 7, 5, 8, 10, 6} 
        query indexes = {3, 6, 1}
Output: 8 -1 7 
Explanation : 
For the 1st query index is 3, element is 7 and 
the next greater element at its right is 8 

For the 2nd query index is 6, element is 10 and 
there is no element greater than 10 at right, 
so print -1.

For the 3rd query index is 1, element is 4 and
the next greater element at its right is 7.

Normal Approach: A normal approach will be for every query to move in a loop from index to n and find out the next greater element and print it, but this in the worst case will take n iterations, which is a lot if the number of queries are high. 
Time Complexity: O(n^2) 
Auxiliary Space>: O(1)
Efficient Approach:
An efficient approach is based on next greater element. We store the index of the next greater element in an array and for every query process, answer the query in O(1) that will make it more efficient. 
But to find out the next greater element for every index in array there are two ways. 
One will take o(n^2) and O(n) space which will be to iterate from I+1 to n for each element at index I and find out the next greater element and store it.
But the more efficient one will be to use stack, where we use indexes to compare and store in next[] the next greater element index.
1) Push the first index to stack. 
2) Pick rest of the indexes one by one and follow following steps in loop. 
….a) Mark the current element as i. 
….b) If stack is not empty, then pop an index from stack and compare a[index] with a[I]. 
….c) If a[I] is greater than the a[index], then a[I] is the next greater element for the a[index]. 
….d) Keep popping from the stack while the popped index element is smaller than a[I]. a[I] becomes the next greater element for all such popped elements 
….g) If a[I] is smaller than the popped index element, then push the popped index back.
3) After the loop in step 2 is over, pop all the index from stack and print -1 as next index for them.

C++

// C++ program to print
// next greater number
// of Q queries
#include <bits/stdc++.h>
using namespace std;

// array to store the next
// greater element index
void next_greatest(int next[],
                   int a[], int n)
{
    // use of stl
    // stack in c++
    stack<int> s;

    // push the 0th
    // index to the stack
    s.push(0);

    // traverse in the
    // loop from 1-nth index
    for (int i = 1; i < n; i++)
    {

        // iterate till loop is empty
        while (!s.empty()) {

            // get the topmost
            // index in the stack
            int cur = s.top();

            // if the current element is
            // greater than the top indexth
            // element, then this will be
            // the next greatest index
            // of the top indexth element
            if (a[cur] < a[i])
            {

                // initialise the cur
                // index position's
                // next greatest as index
                next[cur] = i;

                // pop the cur index
                // as its greater
                // element has been found
                s.pop();
            }

            // if not greater
            // then break
            else
                break;
        }
        // push the i index so that its
        // next greatest can be found
        s.push(i);
    }

    // iterate for all other
    // index left inside stack
    while (!s.empty())
    {
        int cur = s.top();

        // mark it as -1 as no
        // element in greater
        // then it in right
        next[cur] = -1;

        s.pop();
    }
}

// answers all
// queries in O(1)
int answer_query(int a[], int next[],
                 int n, int index)
{
    // stores the next greater
    // element positions
    int position = next[index];

    // if position is -1 then no
    // greater element is at right.
    if (position == -1)
        return -1;

    // if there is a index that
    // has greater element
    // at right then return its
    // value as a[position]
    else
        return a[position];
}

// Driver Code
int main()
{

    int a[] = {3, 4, 2, 7,
               5, 8, 10, 6 };

    int n = sizeof(a) / sizeof(a[0]);

    // initializes the
    // next array as 0
    int next[n] = { 0 };

    // calls the function
    // to pre-calculate
    // the next greatest
    // element indexes
    next_greatest(next, a, n);

    // query 1 answered
    cout << answer_query(a, next, n, 3) << " ";

    // query 2 answered
    cout << answer_query(a, next, n, 6) << " ";

    // query 3 answered
    cout << answer_query(a, next, n, 1) << " ";
}

Java

// Java program to print
// next greater number
// of Q queries
import java.util.*;

class GFG
{
    public static int[] findGreaterElements(int arr[]) {
        int ans[] = new int[arr.length];
        Stack<Integer> s = new Stack<>();

        s.push(arr[arr.length - 1]);
        ans[arr.length - 1] = -1;

        for (int i = arr.length - 2; i >= 0; i--) {
            int curr = arr[i];

            if (s.isEmpty()) {
                ans[i] = -1;
            } else {
                if (s.peek() <= curr) {
                    while (s.peek() <= curr) {
                        s.pop();
                        if (s.isEmpty()) {
                            break;
                        }
                    }
                    if (s.isEmpty()) {
                        ans[i] = -1;
                    } else {
                        ans[i] = s.peek();
                    }
                } else {
                    ans[i] = s.peek();
                }
                s.push(curr);
            }
        }
        return ans;
    }
    // Driver Code
    public static void main(String[] args) {
        int arr[] = {3, 4, 2, 7,
                     5, 8, 10, 6};
        int query[] = {3, 6, 1};
        int ans[] = findGreaterElements(arr);

        // getting output array
        // with next greatest elements
        for(int i = 0; i < query.length; i++) {
            // displaying the next greater
            // element for given set of queries
            System.out.print(ans[query[i]] + " ");
        }
    }
}

// This code was contributed
// by Raj Suvariya

Python3

# Python3 program to print
# next greater number
# of Q queries

# array to store the next
# greater element index
def next_greatest(next, a, n):

    # use of stl
    # stack in c++
    s = []

    # push the 0th
    # index to the stack
    s.append(0);

    # traverse in the
    # loop from 1-nth index
    for  i in range(1, n):

        # iterate till loop is empty
        while (len(s) != 0):

            # get the topmost
            # index in the stack
            cur = s[-1]

            # if the current element is
            # greater than the top indexth
            # element, then this will be
            # the next greatest index
            # of the top indexth element
            if (a[cur] < a[i]):

                # initialise the cur
                # index position's
                # next greatest as index
                next[cur] = i;

                # pop the cur index
                # as its greater
                # element has been found
                s.pop();

            # if not greater
            # then break
            else:
                break;

        # push the i index so that its
        # next greatest can be found
        s.append(i);

    # iterate for all other
    # index left inside stack
    while(len(s) != 0):
        cur = s[-1]

        # mark it as -1 as no
        # element in greater
        # then it in right
        next[cur] = -1;
        s.pop();

# answers all
# queries in O(1)
def answer_query(a, next, n, index):

    # stores the next greater
    # element positions
    position = next[index];

    # if position is -1 then no
    # greater element is at right.
    if(position == -1):
        return -1;

    # if there is a index that
    # has greater element
    # at right then return its
    # value as a[position]
    else:
        return a[position];

# Driver Code
if __name__=='__main__':

    a = [3, 4, 2, 7, 5, 8, 10, 6 ]
    n = len(a)

    # initializes the
    # next array as 0
    next=[0 for i in range(n)]

    # calls the function
    # to pre-calculate
    # the next greatest
    # element indexes
    next_greatest(next, a, n);

    # query 1 answered
    print(answer_query(a, next, n, 3), end = ' ')

    # query 2 answered
    print(answer_query(a, next, n, 6), end = ' ')

    # query 3 answered
    print(answer_query(a, next, n, 1), end = ' ')

# This code is contributed by rutvik_56.

C#

// C# program to print next greater
// number of Q queries
using System;
using System.Collections.Generic;

class GFG
{
public static int[] query(int[] arr,
                          int[] query)
{
    int[] ans = new int[arr.Length]; // this array contains
                                     // the next greatest
                                     // elements of all the elements
    Stack<int> s = new Stack<int>();

    // push the 0th index to the stack
    s.Push(arr[0]);
    int j = 0;

    // traverse rest of the array
    for (int i = 1; i < arr.Length; i++)
    {
        int next = arr[i];

        if (s.Count > 0)
        {
            // get the topmost element in the stack
            int element = s.Pop();

            /* If the popped element is smaller
            than next, then
            a) store the pair
            b) keep popping while
            elements are smaller and
            stack is not empty */
            while (next > element)
            {
                ans[j] = next;
                j++;
                if (s.Count == 0)
                {
                    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 -1 for them */
    while (s.Count > 0)
    {
        int element = s.Pop();
        ans[j] = -1;
        j++;
    }

    // return the next greatest array
    return ans;
}

// Driver Code
public static void Main(string[] args)
{
    int[] arr = new int[] {3, 4, 2, 7, 5, 8, 10, 6};
    int[] query = new int[] {3, 6, 1};
    int[] ans = GFG.query(arr, query);

    // getting output array
    // with next greatest elements
    for (int i = 0; i < query.Length; i++)
    {
        // displaying the next greater
        // element for given set of queries
        Console.Write(ans[query[i]] + " ");
    }
}
}

// This code is contributed by Shrikant13

Javascript

<script>

// JavaScript program to print
// next greater number
// of Q queries

// array to store the next
// greater element index
function next_greatest(next, a, n)
{
    // use of stl
    // stack in c++
    var s = [];

    // push the 0th
    // index to the stack
    s.push(0);

    // traverse in the
    // loop from 1-nth index
    for (var i = 1; i < n; i++)
    {

        // iterate till loop is empty
        while (s.length!=0) {

            // get the topmost
            // index in the stack
            var cur = s[s.length-1];

            // if the current element is
            // greater than the top indexth
            // element, then this will be
            // the next greatest index
            // of the top indexth element
            if (a[cur] < a[i])
            {

                // initialise the cur
                // index position's
                // next greatest as index
                next[cur] = i;

                // pop the cur index
                // as its greater
                // element has been found
                s.pop();
            }

            // if not greater
            // then break
            else
                break;
        }
        // push the i index so that its
        // next greatest can be found
        s.push(i);
    }

    // iterate for all other
    // index left inside stack
    while (s.length!=0)
    {
        var cur = s[s.length-1];

        // mark it as -1 as no
        // element in greater
        // then it in right
        next[cur] = -1;

        s.pop();
    }
}

// answers all
// queries in O(1)
function answer_query(a, next, n, index)
{
    // stores the next greater
    // element positions
    var position = next[index];

    // if position is -1 then no
    // greater element is at right.
    if (position == -1)
        return -1;

    // if there is a index that
    // has greater element
    // at right then return its
    // value as a[position]
    else
        return a[position];
}

// Driver Code
var a = [3, 4, 2, 7,
           5, 8, 10, 6];
var n = a.length;
// initializes the
// next array as 0
var next = Array(n).fill(0);
// calls the function
// to pre-calculate
// the next greatest
// element indexes
next_greatest(next, a, n);
// query 1 answered
document.write( answer_query(a, next, n, 3) + " ");
// query 2 answered
document.write( answer_query(a, next, n, 6) + " ");
// query 3 answered
document.write( answer_query(a, next, n, 1) + " ");



</script>

Output

8 -1 7 

Time complexity: max(O(n), O(q)), O(n) for pre-processing the next[] array and O(1) for every query.
Auxiliary Space: O(n)
This article is contributed by Raja Vikramaditya(raj). If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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