496. Next Greater Element I

class Solution {
public:
    vector<int> nextGreaterElement(vector<int>& nums1, vector<int>& nums2) {
        unordered_map<int, int> greater;
        stack<int> left;
        
        for (auto n : nums2) {
            while (!left.empty() && left.top() < n) {
                greater[left.top()] = n;
                left.pop();
            }
            left.push(n);
        }
        
        vector<int> ans(nums1.size());
        
        for (int i = 0; i < nums1.size(); ++i) {
            ans[i] = greater.count(nums1[i]) ? greater[nums1[i]] : -1;
        }
        return ans;
    }
};