Water Area

• Time : O(n)
• Space : O(1)

int waterArea(vector<int> h) {
    int res = 0, l = 0, r = h.size() - 1, mi = 0;
    while(l < r) {
        while(l <= r and mi >= h[l]) l++;
        while(l <= r and mi >= h[r]) r--;
        if(l <= r) {
            res += (min(h[l], h[r]) - mi) * (r - l + 1);
            mi = min(h[l], h[r]);
        }
    }
    
    return res - accumulate(begin(h), end(h), 0);
}