Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it can trap after raining.
Example 1:
Input: height = [0,1,0,2,1,0,1,3,2,1,2,1]
Output: 6
Explanation: The elevation map [0,1,0,2,1,0,1,3,2,1,2,1] traps 6 units of rain water. Water fills the valleys between the bars: 1 unit between indices 1-3, 4 units between indices 3-7, and 1 unit between indices 8-10.
Example 2:
Input: height = [4,2,0,3,2,5]
Output: 9
Explanation: Water fills the gap between the tall bars at indices 0 and 5, trapping 2 + 4 + 1 + 2 = 9 units total.
Example 3:
Input: height = [1,0,1]
Output: 1
Explanation: One unit of water is trapped between the two bars of height 1.
n == height.length1 <= n <= 2 * 10⁴0 <= height[i] <= 10⁵