题目
重点题目，深刻理解！！！

解法1：备忘录

O(N) + O(N)

class Solution {public int trap(int[] height) {int n = height.length, res = 0;int[] lMax = new int[n];int[] rMax = new int[n];lMax[0] = height[0];rMax[n - 1] = height[n - 1];for (int i = 1; i < n; ++i) {lMax[i] = Math.max(lMax[i - 1], height[i]);}for (int i = n - 2; i >=0; --i) {rMax[i] = Math.max(rMax[i + 1], height[i]);}for (int i = 0; i < n; ++i) {res += Math.min(lMax[i], rMax[i]) - height[i];}return res;}
}

解法2：双指针

O(N) + O(1)

class Solution {public int trap(int[] height) {int n = height.length;int left = 0, right = n - 1, res = 0, lMax = height[0], rMax = height[n - 1];while (left <= right) {lMax = Math.max(lMax, height[left]);rMax = Math.max(rMax, height[right]);if (lMax <= rMax) {res += lMax - height[left];++left;} else {res += rMax - height[right];--right;}}return res;}
}