exist = function(board, word) {
    for(let row = 0; row < board.length; row++) {
        for(let col = 0; col < board[0].length; col++){
            if (dfs(board, row, col, word, 0)) return true;
        }
    }

    return false;
}
https://leetcode.com/problems/word-search/
Time O(N * 3^L) | Space O(L)
@param {character[][]} board
@param {string} word
@return {boolean}