LeetCode Online Judge 题目C# 练习 - Trapping Rain Water

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.

public static int TrappingRainWater(int[] A)
{
if (A.Length <= 2)
return 0;

int h = 0;
// find the highest index
for (int i = 0; i < A.Length; i++)
{
if (A[i] > A[h])
h = i;
}

int water = 0;
int p = 0;
// calculate from left to highest
for (int i = 1; i < h; i++)
{
if (A[p] > A[i])
water += A[p] - A[i];
else
p = i;
}

p = A.Length - 1;
// calculate from right to highest
for (int i = A.Length - 2; i > h; i--)
{
if (A[p] > A[i])
water += A[p] - A[i];
else
p = i;
}

return water;
}

代码分析：

　　这是我一开始复习就在别人的面经上看到的题，数数手指也快2年了，唉。。。。什么时候才能给我OFFER啊？？

　　1. 找到最高的index

　　2. 从左往右算water 的体积（面积）。p = 0， 如果A[i] < A[p]，差值就可以承载水，如果A[i] >= A[p] ，p = i

　　3. 从右往左在加一次water。 同理。