Add trappedWater class to calculate trapped water

[gauravbisen1]

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
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• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.05%. Comparing base (48ba1ae) to head (ea0f97e).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6924      +/-   ##
============================================
- Coverage     78.05%   78.05%   -0.01%     
+ Complexity     6466     6465       -1     
============================================
  Files           738      738              
  Lines         21585    21585              
  Branches       4220     4220              
============================================
- Hits          16849    16848       -1     
  Misses         4064     4064              
- Partials        672      673       +1     

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