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Rain Water Trapped

Problem Description

Given an integer array A of 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.

Problem Constraints

1 <= |A| <= 100000

Input Format

The only argument given is integer array A.

Output Format

Return the total water it is able to trap after raining.

Example Input

Input 1:

``` A = [0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1]
```

Input 2:

``` A = [1, 2]
```

Example Output

Output 1:

``` 6
```

Output 2:

``` 0
```

Example Explanation

Explanation 1:

```
In this case, 6 units of rain water (blue section) are being trapped.
```

Explanation 2:

``` No water is trapped.
```

NOTE: You only need to implement the given function. Do not read input, instead use the arguments to the function. Do not print the output, instead return values as specified. Still have a question? Checkout Sample Codes for more details.
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