79. 单词搜索（算法200题-39）

var exist = function (board, word) {
    if (!board.length) return false
    if (!word.length) return true
    let row = board[0].length,
        col = board.length
    for (let i = 0; i < row; i++) {
        for (let j = 0; j < col; j++) {
            const res = backtrack(i, j, 0)
            if (res) return res
        }
    }
    return false
    function backtrack(i, j, cur) {
        if(i >= row || i < 0) return false
        if(j >= col || j < 0) return false
        const curStr = board[j][i]
        if(curStr !== word[cur]) return false
        if(cur === word.length - 1) return true
        board[j][i] = null
        const res = backtrack(i + 1, j, cur + 1) ||
            backtrack(i - 1, j, cur + 1) ||
            backtrack(i, j + 1, cur + 1) ||
            backtrack(i, j - 1, cur + 1);
        board[j][i] = curStr
        return res
    }
};