NextGreaterElementI.cpp

#include <iostream>
#include <stack>
#include <vector>
#include <unordered_map>
using namespace std;

vector<int> nextGreaterElement(vector<int> &nums1, vector<int> &nums2)
{
    unordered_map<int, int> m;
    stack<int> s;

    for (int i = nums2.size() - 1; i >= 0; i--)
    {
        while (s.size() > 0 && s.top() <= nums2[i]) // removing invlaid values
        {
            s.pop();
        }

        if (s.empty())
        {
            m[nums2[i]] = -1;
        }
        else
        {
            m[nums2[i]] = s.top();
        }

        s.push(nums2[i]);
    }

    vector<int> ans;

    for (int i = 0; i < nums1.size(); i++)
    {
        ans.push_back(m[nums1[i]]);
    }

    return ans;
}

int main()
{
    vector<int> arr1 = {4, 1, 2};
    vector<int> arr2 = {1, 3, 4, 2};

    vector<int> ans = nextGreaterElement(arr1, arr2);

    for (int val : ans)
    {
        cout << val << " ";
    }
    cout << endl;

    return 0;
}