739. 每日温度

class Solution {
public:
    vector<int> dailyTemperatures(vector<int>& temperatures) {
        int n = temperatures.size();
        stack<int> sta;
        vector<int> res(n, 0);
        sta.push(0);
        for (int i = 0; i < n; i ++)
        {
            while (!sta.empty() && temperatures[i] > temperatures[sta.top()])
            {
                res[sta.top()] = i - sta.top();
                sta.pop();
            }
            sta.push(i);
        }
        return res;
    }
};

496. 下一个更大元素 I

用一个哈希表记录 nums1 的数字和下标的关系

class Solution {
public:
    vector<int> nextGreaterElement(vector<int>& nums1, vector<int>& nums2) {
        if (nums1.size() == 0) return {};
        unordered_map<int, int> umap; // (val, idx)
        stack<int> sta;
        vector<int> res(nums1.size(), -1);
        for (int i = 0; i < nums1.size(); i ++)
            umap[nums1[i]] = i;

        sta.push(0);
        for (int i = 1; i < nums2.size(); i ++)
        {
            while (sta.size() && nums2[i] > nums2[sta.top()])
            {
                if (umap.count(nums2[sta.top()]))
                {
                    int idx = umap[nums2[sta.top()]];
                    res[idx] = nums2[i];
                }
                sta.pop();
            }
            sta.push(i);
        }
        return res;
    }
};