42.Trapping Rain Water

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

class Solution {
public:
    int trap(vector<int>& height) {
        int l = 0, r = height.size() - 1, level = 0, res = 0;
        while (l < r) {
            int lower = height[(height[l] < height[r]) ? l++ : r--];
            level = max(level, lower);
            res += level - lower;
        }
        return res;
    }
};