79. Word Search
給你一個二維字元陣列board和一個字串word，在board中搜尋是否存在字元可以串接組成
word。
Example 1:
https://assets.leetcode.com/uploads/2020/11/04/word2.jpg
Input: board = [["A","B","C","E"],["S","F","C","S"],["A","D","E","E"]], word
= "ABCCED"
Output: true
Example 2:
https://assets.leetcode.com/uploads/2020/10/15/word3.jpg
Input: board = [["A","B","C","E"],["S","F","C","S"],["A","D","E","E"]], word
= "ABCB"
Output: false
思路：
1.看起來可以用字典樹或回溯法，但是JAVA要手刻字典樹所以我決定用回溯法搞定。
2.把board的每個點當成起點，如果匹配word長度就+1，如果長度等於word就返回true。
3.走過的格子要標記成已走過，如果下次遇到已走過的格子就跳過。
JavaCode：

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不知道有沒有辦法加速

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