Solution.ts

function exist(board: string[][], word: string): boolean {
    const [m, n] = [board.length, board[0].length];
    const dirs = [-1, 0, 1, 0, -1];
    const dfs = (i: number, j: number, k: number): boolean => {
        if (k === word.length - 1) {
            return board[i][j] === word[k];
        }
        if (board[i][j] !== word[k]) {
            return false;
        }
        const c = board[i][j];
        board[i][j] = '0';
        for (let u = 0; u < 4; ++u) {
            const [x, y] = [i + dirs[u], j + dirs[u + 1]];
            const ok = x >= 0 && x < m && y >= 0 && y < n;
            if (ok && board[x][y] !== '0' && dfs(x, y, k + 1)) {
                return true;
            }
        }
        board[i][j] = c;
        return false;
    };
    for (let i = 0; i < m; ++i) {
        for (let j = 0; j < n; ++j) {
            if (dfs(i, j, 0)) {
                return true;
            }
        }
    }
    return false;
}