题目：

题解：

class Solution {public int trap(int[] height) {int n = height.length;if (n == 0) {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 ans = 0;for (int i = 0; i < n; ++i) {ans += Math.min(leftMax[i], rightMax[i]) - height[i];}return ans;}
}