42. 接雨水

class Solution:
    def trap(self, height: List[int]) -> int:
        n = len(height)
        if n < 3:
            return 0

        left_max = [height[0]] * n
        for i in range(1, n):
            left_max[i] = max(left_max[i - 1], height[i])

        right_max = [height[n - 1]] * n
        for i in range(n - 2, -1, -1):
            right_max[i] = max(right_max[i + 1], height[i])

        res = 0
        for i in range(n):
            res += min(left_max[i], right_max[i]) - height[i]
        return res

class Solution {
    public int trap(int[] height) {
        int n;
        if ((n = height.length) < 3) return 0;

        int[] leftMax = new int[n];
        leftMax[0] = height[0];
        for (int i = 1; i < n; ++i) {
            leftMax[i] = Math.max(leftMax[i - 1], height[i]);
        }

        int[] rightMax = new int[n];
        rightMax[n - 1] = height[n - 1];
        for (int i = n - 2; i >= 0; --i) {
            rightMax[i] = Math.max(rightMax[i + 1], height[i]);
        }

        int res = 0;
        for (int i = 0; i < n; ++i) {
            res += Math.min(leftMax[i], rightMax[i]) - height[i];
        }
        return res;
    }
}