题目原型：
Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after raining.
For example,
Given [0,1,0,2,1,0,1,3,2,1,2,1], return 6.
The above elevation map is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped. Thanks MarcZ http://www.2cto.com/kf/ware/vc/" target="_blank" class="keylink">vcyBmb3IgY29udHJpYnV0aW5nIHRoaXMgaW1hZ2UhPC9wPgo8cD4Ku/mxvsu8wrejujwvcD4KPHA+CtXStb3X7rTztcTK/bXEy/fS/aOsyLu689LAtMvOqr3no6y31rHwz/LX87rNz/LT0smow+ijrNP2tb21scewuN+2yLHIx7DD5mxlZnRNYXhJbmRleLjftsjQobXE1PK8xsvjw+a7/aOst7TWrqOsuPzQwmxlZnRNYXhJbmRleKO7tNPT0s/y1/OjrM2swO2joTwvcD4KPHA+CjxwcmUgY2xhc3M9"brush:java;"> public int trap(int[] A) { if(A.length<=2) return 0; int ret = 0;//结果 //找到最大的数的index int maxIndex = 0; for(int i = 1;i