42. Trapping Rain Water.cpp

class Solution
{
public:
    int trap(vector<int> &height)
    {
        int n = height.size();
        if (n == 0)
            return 0;
        vector<int> lmax(n), rmax(n);
        lmax[0] = height[0];
        rmax[n - 1] = height[n - 1];
        for (int i = 1; i < n; i++)
            lmax[i] = max(lmax[i - 1], height[i]);
        for (int i = n - 2; i >= 0; i--)
            rmax[i] = max(rmax[i + 1], height[i]);
        int ans(0);
        for (int i = 1; i < n - 1; i++)
        {
            ans += min(lmax[i], rmax[i]) - height[i];
        }
        return ans;
    }
};