单调栈

int main() {
    vector<int> nums{ 5,4,2,6,7,3,9 };
    int length = nums.size();

    stack<vector<int>>sk1;
    vector<int>rightFirstMax(length, 0);
    for (int index = length - 1; index >= 0; index--) {
        while (!sk1.empty() && nums[index] > sk1.top()[0]) {
            sk1.pop();
        }
        if (!sk1.empty()) {
            rightFirstMax[index] = sk1.top()[1];
        }
        vector<int>temp(2, 0);
        temp[0] = nums[index];
        temp[1] = index;
        sk1.push(temp);
    }

    vector<int>leftFirstMin(length, 0);
    stack<vector<int>>sk2;
    for (int index = 0; index < length; index++) {
        while (!sk2.empty() && nums[index] < sk2.top()[0]) {
            sk2.pop();
        }
        if (!sk2.empty()) {
            leftFirstMin[index] = sk2.top()[1];
        }
        vector<int>temp(2, 0);
        temp[0] = nums[index];
        temp[1] = index;
        sk2.push(temp);
    }

    int res = 0;
    for (int i = 0; i < length; ++i) {
        int temp = rightFirstMax[i] * leftFirstMin[i];
        if (temp > res) {
            res = temp;
        }
    }
    cout << res;
    return 0;
}