Word Search

Given an m x n grid of characters board and a string word, return true if word exists in the grid.

The word can be constructed from letters of sequentially adjacent cells, where adjacent cells are horizontally or vertically neighboring. The same letter cell may not be used more than once.

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Example 1

A B C E
S F C S
A D E E

Input: board = [["A","B","C","E"],["S","F","C","S"],["A","D","E","E"]], word = "ABCCED"
Output: true
Explanation: A->B->C->C->E->D traces a valid path through the grid.

Example 2

A B C E
S F C S
A D E E

Input: board = [["A","B","C","E"],["S","F","C","S"],["A","D","E","E"]], word = "SEE"
Output: true
Explanation: S->E->E at (1,3)->(2,3)->(2,2) is a valid adjacent path.

Example 3

A B C E
S F C S
A D E E

Input: board = [["A","B","C","E"],["S","F","C","S"],["A","D","E","E"]], word = "ABCB"
Output: false
Explanation: B at (0,1) cannot be reused; no valid path for ABCB exists.

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Constraints

• m == board.length
• n == board[i].length
• 1 <= m, n <= 6
• 1 <= word.length <= 15
• board and word consist of only lowercase and uppercase English letters.