Largest Rectangle in Histogram

class Solution {
public:
    int largestRectangleArea(vector<int> &h) {
   
    stack<int> stk;
    int index = 0, maxArea = 0;
    h.push_back(0);
    
    while(index < h.size()) {
        
        if(stk.empty() || h[stk.top()] <= h[index]){
            
            stk.push(index);
            index++;    
        }
        
        else {
            int prev = stk.top();
            stk.pop();
            maxArea = max(maxArea, h[prev] * ( stk.empty() ? index :index-stk.top() - 1 ));
        }
    }
    return maxArea;
    }
};