Improve space complexity to O(1) in WordSearch. (#7308)

* Modifying space complexity to O(1).

* Fix formatting using clang-format.

* Fix checkstyle violations.

* Fix checkstyle violations and code formatting.

* Remove unused fields reported by SpotBugs.

* Remove unused fields and comments.

* Remove unused field reported by SpotBugs.

* Fix PMD collapsible if statement.

* Fix indentation to satisfy clang-format.

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Co-authored-by: Deniz Altunkapan <deniz.altunkapan@outlook.com>