leetcode 496 next larger element I

 

Stack do monotonically decreasing, time O (n), spaceO (n) requires a hash map

class Solution {
 public : 
    Vector < int > nextGreaterElement (Vector < int > & nums1, Vector < int > & nums2) {
         // with a monotonically decreasing from the stack top to the stack bottom of the stack do this problem, traversing from right to left ;
         // 1. If the current element is greater than the top element, described on the right of the first element is larger than the x stack;
         // 2. If the current element is less than the top element, the top element pop put off, and then repeat the line guidance stack is empty;
         //    because even after the next greater element of looking at the very least is the current element, because the current element from
         //    closer after the elements, and greater value; 
        int len1 = nums1.size ();
         IF (LEN1 == 0 ) return {}; 
        Stack < int > s;
        unordered_map<int,int> m;//map<num2,nextgreater>
        vector<int> res;
        for(int i=nums2.size()-1;i>=0;i--){
            
            while(!s.empty()){
                if(s.top()>nums2[i]){
                    m[nums2[i]]=s.top();break;
                }else{
                    s.pop();
                }
            }
            if(s.empty()){
                m[nums2[i]]=-1;
            }
            s.push(nums2[i]);  
        }
        
        for(int i=0;i<len1;i++){
            res.push_back(m[nums1[i]]);
        }
        return res;
    }
};