Problem

Given a 2D board of characters and a string word, return True if the 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 in the word.

Examples

Example 1:

Input: board = [['A','B','C','E'],
               ['S','F','C','S'],
               ['A','D','E','E']],
       word = 'ABCCED'
Output: True

Example 2:

Input: board = [['A','B','C','E'],
               ['S','F','C','S'],
               ['A','D','E','E']],
       word = 'SEE'
Output: True

Constraints

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