
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.
m == board.lengthn = board[i].length1 <= m, n <= 61 <= word.length <= 15board and word consists of only lowercase and uppercase English letters.Follow up: Could you use search pruning to make your solution faster with a larger board ?