* @param {character[][]} board
const exist = function (board, word) {
const n = board[0].length;
for (let i = 0; i < m; i++) {
for (let j = 0; j < n; j++) {
if (wordSearch(i, j, 0)) {
function wordSearch (i, j, k) {
if (i < 0 || j < 0 || i >= m || j >= n || word[k] !== board[i][j]) {
// 找到最后一个字符,返回 true,为递归的终止条件
if (k === word.length - 1) {
const temp = board[i][j];
const res = wordSearch(i + 1, j, k + 1) ||
wordSearch(i - 1, j, k + 1) ||
wordSearch(i, j + 1, k + 1) ||
wordSearch(i, j - 1, k + 1);