思路：循环，所以从后往前执行两次单调栈

    1.     vector<int> nextGreaterElements(vector<int>& nums) {
    2.         if(nums.empty())return {};
    3.         int len = nums.size();
    4.         vector<int> ans(len, 0);
    5.         stack<int> s;
    6.         for(int i = len - 1;i >= 0;--i){
    7.             while(!s.empty() && nums[i] >= s.top())s.pop();
    8.             if(s.empty())ans[i] = -1;
    9.             else ans[i] = s.top();
    10.             s.push(nums[i]);
    11.         }
    12.         for(int i = len - 1;i >= 0;--i){
    13.             while(!s.empty() && nums[i] >= s.top())s.pop();
    14.             if(s.empty())ans[i] = -1;
    15.             else ans[i] = s.top();
    16.             s.push(nums[i]);
    17.         }
    18.         return ans;
    19.     }