Solution.ts

/**
 Do not return anything, modify board in-place instead.
 */
function solve(board: string[][]): void {
    let m = board.length,
        n = board[0].length;
    if (m < 3 || n < 3) return;
    let visited = Array.from({ length: m }, v => new Array(n).fill(false));
    // 第一行，最后一行， 第一列， 最后一列
    for (let i of [0, m - 1]) {
        for (let j = 0; j < n; ++j) {
            if (board[i][j] == "X") {
                visited[i][j] = true;
            } else {
                dfs(board, i, j, visited, true);
            }
        }
    }
    for (let i = 0; i < m; ++i) {
        for (let j of [0, n - 1]) {
            if (board[i][j] == "X") {
                visited[i][j] = true;
            } else {
                dfs(board, i, j, visited, true);
            }
        }
    }
    for (let i = 1; i < m - 1; ++i) {
        for (let j = 1; j < n - 1; ++j) {
            !visited[i][j] && dfs(board, i, j, visited);
        }
    }
}

function dfs(
    board: string[][],
    i: number,
    j: number,
    visited: boolean[][],
    edge = false
): void {
    let m = board.length,
        n = board[0].length;
    if (i < 0 || i > m - 1 || j < 0 || j > n - 1 || visited[i][j]) {
        return;
    }

    visited[i][j] = true;
    if (board[i][j] == "X") {
        return;
    }
    if (!edge) {
        board[i][j] = "X";
    }
    for (let [dx, dy] of [
        [0, 1],
        [0, -1],
        [1, 0],
        [-1, 0],
    ]) {
        let x = i + dx,
            y = j + dy;
        dfs(board, x, y, visited, edge);
    }
}