For example,
Given [0,1,0,2,1,0,1,3,2,1,2,1], return 6.
[分析] 此题两种解法均参考 http://blog.csdn.net/linhuanmars/article/details/20888505,详细分析可查看原文。摘出关键点拨:思路 1 从两边各扫描一次得到我们需要维护的变量,通常适用于当前元素需要两边元素来决定的问题,非常类似的题目是Candy, 思路2 是向中间夹逼时能确定接下来移动一侧窗口不可能使结果变得更好,所以每次能确定移动一侧指针,直到相遇为止。这种方法带有一些贪心,用到的有Two Sum,Container With Most Water。
public class Solution { // method 1: time O(N), space O(N), need 2 scans public int trap(int[] height) { if (height == null || height.length <= 2) return 0; int N = height.length; int[] bar = new int[N]; int max = 0; for (int i = 0; i < N; i++) { bar[i] = max; max = Math.max(max, height[i]); } max = 0; int res = 0; for (int i = N - 1; i >= 0; i--) { bar[i] = Math.min(max, bar[i]); max = Math.max(max, height[i]); res += bar[i] - height[i] > 0 ? bar[i] - height[i] : 0; } return res; } // method 2: time O(N), only one scan public int trap2(int[] height) { if (height == null || height.length <= 2) return 0; int l = 0, r = height.length - 1; int min; int res = 0; while (l < r) { min = Math.min(height[l], height[r]); if (height[l] == min) { l++; while (l < r && height[l] < min) { res += min - height[l]; l++; } } else { r--; while (l < r && height[r] < min) { res += min - height[r]; r--; } } } return res; } }