单调栈

 1 int main() {
 2     vector<int> nums{ 5,4,2,6,7,3,9 };
 3     int length = nums.size();
 4 
 5     stack<vector<int>>sk1;
 6     vector<int>rightFirstMax(length, 0);
 7     for (int index = length - 1; index >= 0; index--) {
 8         while (!sk1.empty() && nums[index] > sk1.top()[0]) {
 9             sk1.pop();
10         }
11         if (!sk1.empty()) {
12             rightFirstMax[index] = sk1.top()[1];
13         }
14         vector<int>temp(2, 0);
15         temp[0] = nums[index];
16         temp[1] = index;
17         sk1.push(temp);
18     }
19 
20     vector<int>leftFirstMin(length, 0);
21     stack<vector<int>>sk2;
22     for (int index = 0; index < length; index++) {
23         while (!sk2.empty() && nums[index] < sk2.top()[0]) {
24             sk2.pop();
25         }
26         if (!sk2.empty()) {
27             leftFirstMin[index] = sk2.top()[1];
28         }
29         vector<int>temp(2, 0);
30         temp[0] = nums[index];
31         temp[1] = index;
32         sk2.push(temp);
33     }
34 
35     int res = 0;
36     for (int i = 0; i < length; ++i) {
37         int temp = rightFirstMax[i] * leftFirstMin[i];
38         if (temp > res) {
39             res = temp;
40         }
41     }
42     cout << res;
43     return 0;
44 }