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.
Input:
[0,1,0,2,1,0,1,3,2,1,2,1]
Output: 6
Input:
[4,2,0,3,2,5]
Output: 9
Input:
[1,2,3,4]
Output: 0
Your program must print a single integer representing the total trapped water so it can be compared with the expected output.
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Discuss brute-force, prefix max arrays, two-pointer optimization, and stack-based approaches.