trapping-rain-water.md

<p>Given <em>n</em> 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.</p>

<p><img src="https://assets.leetcode.com/uploads/2018/10/22/rainwatertrap.png" style="width: 412px; height: 161px;" /><br />
<small>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. <strong>Thanks Marcos</strong> for contributing this image!</small></p>

<p><strong>Example:</strong></p>

<pre>
<strong>Input:</strong> [0,1,0,2,1,0,1,3,2,1,2,1]
<strong>Output:</strong> 6</pre>