Rain Water Trapped

• Time : O(n)
• Space : O(1)

int Solution::trap(const vector<int> &A) {
    int l = 0, r = A.size() - 1, res = 0, mi = 0;
    while(l < r) {
        if(A[l] > mi and A[r] > mi) {
            res += (r - l + 1) * (min(A[l],A[r]) - mi);
            mi = min(A[l],A[r]);
            res -= mi;
            if(A[l] < A[r]) l++;
            else r--;
        } else if(A[l] <= mi and A[r] >= mi) {
            res -= A[l++];
        } else if(A[r] <= mi and A[l] >= mi) {
            res -= A[r--];
        }
    }
    return res - mi;
}