[LeetCode] Largest Rectangle in Histogram

This link has a very neat code, which is rewritten below using stack since the push and pop operations of it are O(1) time, while the pop_back and push_back of vector tend to be more time-consuming. This is also verified by the running time of the code on the OJ: stack version is generally 4ms to 8ms faster than vector version.

class Solution {
public:
    int largestRectangleArea(vector<int>& height) {
        height.push_back(0);
        int n = height.size(), area = 0;
        stack<int> indexes;
        for (int i = 0; i < n; i++) {
            while (!indexes.empty() && height[indexes.top()] > height[i]) {
                int h = height[indexes.top()]; indexes.pop();
                int l = indexes.empty() ? -1 : indexes.top();
                area = max(area, h * (i - l - 1));
            }
            indexes.push(i);
        }
        return area; 
    }
};

Moreover, it would be better to keep height unmodified. So we loop for n + 1 times and manually set the h = 0 when i == n. The code is as follows.

class Solution {
public:
    int largestRectangleArea(vector<int>& height) {
        int n = height.size(), area = 0, h, l;
        stack<int> indexes;
        for (int i = 0; i <= n; i++) {
            while (i == n || (!indexes.empty() && height[indexes.top()] > height[i])) {
                if (i == n && indexes.empty()) h = 0, i++;
                else h = height[indexes.top()], indexes.pop();          
                l = indexes.empty() ? -1 : indexes.top();
                area = max(area, h * (i - l - 1));
            }
            indexes.push(i);
        }
        return area;
    }
};