Update 042._trapping_rain_water.md

Author: 0xkookoo

docs/Leetcode_Solutions/Python/042._trapping_rain_water.md

@@ -1,15 +1,32 @@
-### 42. Trapping Rain Water
+# 42. Trapping Rain Water
 
+**<font color=red>难度: Hard</font>**
 
+## 刷题内容
 
-题目:
-<https://leetcode.com/problems/trapping-rain-water/>
+> 原题连接
 
+* https://leetcode.com/problems/trapping-rain-water/description/
 
-难度:
-Hard
+> 内容描述
 
-思路：
+```
+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.
+```
+
+![](https://github.com/apachecn/awesome-algorithm/blob/master/images/042/rainwatertrap.png)
+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 Marcos for contributing this image!
+```
+Example:
+
+Input: [0,1,0,2,1,0,1,3,2,1,2,1]
+Output: 6
+```
+
+## 解题方案
+
+> 思路 1
+******- 时间复杂度: O(N)******- 空间复杂度: O(1)******
 
 题目有几个特性可用，bar width = 1,然后第一个和最后一个是不能trap water，其次中间的部分能trap多少水是看`左右高度差较低的那个 -  本身的高度`
 
@@ -35,5 +52,4 @@ class Solution(object):
                 water += min_height - height[r]
                 r -= 1
         return water
-
 ```