496. Next Greater Element I

The next greater element of some element x in an array is the first greater element that is to the right of x in the same array.

You are given two distinct 0-indexed integer arrays nums1 and nums2, where nums1 is a subset of nums2.

For each 0 <= i < nums1.length, find the index j such that nums1[i] == nums2[j] and determine the next greater element of nums2[j] in nums2. If there is no next greater element, then the answer for this query is -1.

Return an array ans of length nums1.length such that ans[i] is the next greater element as described above.

class Solution {
public:
    vector<int> nextGreaterElement(vector<int>& nums1, vector<int>& nums2) {
        vector<int> st;
        vector<int> greater(nums2.size(), -1);
        unordered_map<int, int> lookup;
        for(int i = nums2.size() - 1; i >= 0; i--) {
            lookup[nums2[i]] = i;
            
            while(!st.empty() and st.back() < nums2[i])
                st.pop_back();
            if(!st.empty())
                greater[i] = st.back();
            
            st.push_back(nums2[i]);
        }
        
        vector<int> res;
        for(auto n : nums1) {
            res.push_back(greater[lookup[n]]);
        }
        return res;
    }
};