Trapping Rain Water[leetcode]

int trap(int A[], int n) {        if (n == 0) return 0;        int l = 0, r = n - 1;        int lv = A[l], rv = A[r];        int total = A[l] + A[r], rainTotal = total;        while (l != r)        {            if (A[l] < A[r])            {                l++;                total += A[l];                lv = max(lv, A[l]);                rainTotal += lv;            }            else            {                r--;                total += A[r];                rv = max(rv, A[r]);                rainTotal += rv;            }        }        return rainTotal - total;    }