/**
 * https://leetcode.cn/problems/word-search/
 * 
 * 79. 
 *  m x n  board  word  word  true  false 
 * 
 * ""
 * 
 *  1
 * board = [["A","B","C","E"],["S","F","C","S"],["A","D","E","E"]], word = "ABCCED"
 * true
 * 
 *  2
 * board = [["A","B","C","E"],["S","F","C","S"],["A","D","E","E"]], word = "SEE"
 * true
 * 
 *  3
 * board = [["A","B","C","E"],["S","F","C","S"],["A","D","E","E"]], word = "ABCB"
 * false
 * 
 * 
 * m == board.length
 * n = board[i].length
 * 1