[leetcode]Largest Rectangle in Histogram

class Solution {
public:
int largestRectangleArea(vector<int> &height) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
if(height.empty()) return 0;

int N = height.size();
vector<int> left(N, 0);
vector<int> right(N, 0);

stack<int> s1;
s1.push(0);

for(int i = 1; i < N; i++){
if(height[i] > height[s1.top()]){
left[i] = 0;
}else{
while(!s1.empty() && height[i] <= height[s1.top()]){
s1.pop();
}
if(s1.empty()){
left[i] = i;
}else{
left[i] = i-s1.top()-1;
}
}
s1.push(i);
}

stack<int> s2;
s2.push(N-1);

for(int i = N-2; i >= 0; i--){
if(height[i] > height[s2.top()]){
right[i] = 0;
}else{
while(!s2.empty() && height[i] <= height[s2.top()]){
s2.pop();
}
if(s2.empty()){
right[i] = N-i-1;
}else{
right[i] = s2.top()-i-1;
}
}
s2.push(i);
}

for(int i = 0; i < N; i++){
left[i] =(left[i] + right[i] + 1) * height[i];
}

return *max_element(left.begin(), left.end());

}
};