rainwater(IEEEw5).cpp

int Solution::trap(const vector<int> &a) {
    int n = a.size(), ans = 0;
    vector<int> l(n);
    vector<int> r(n);
     
    l[0] = a[0]; 
    for (int i = 1; i < n; i++) 
       left[i] = max(l[i-1], a[i]); 
  
    r[n-1] = a[n-1]; 
    for (int i = n-2; i >= 0; i--) 
       r[i] = max(r[i+1], a[i]); 
  
    for (int i = 0; i < n; i++) 
       ans += min(l[i],r[i]) - a[i]; 
  
    return ans; 
}