Word Search in a 2D board

Problem Given an m x n grid of characters board and a string word, determine whether word can be formed by sequentially adjacent cells. Adjacent means up/down/left/right The same cell may not be used more than once in a single path Output true/false. Constraints 1 <= m, n <= 20 1 <= len(word) <= m*n Example Input: 3 4 A B C E S F C S A D E E ABCCED Output: true Example Input 3 4 A B C E S F C S A D E E ABCCED Output true