Word Search (Grid DFS Backtracking)

Given an m x n grid of characters board and a string word, determine whether word can be formed by sequentially adjacent cells (up, down, left, right). Rules: You may move only to horizontally or vertically adjacent cells. The same cell may be used at most once in a single path. Return true/false. Constraints 1 <= m, n <= 20 1 <= len(word) <= m*n Examples Input: board = [[A,B,C,E], [S,F,C,S], [A,D,E,E]] word = ABCCED Output: true Input same board, word = ABCB Output: false Example Input 3 4 ABCE SFCS ADEE ABCCED Output true