leetcode-130.js

var solve = function(board) {
  for (let row=0; row < board.length; row++) {
    // columna izquierda
    if (board[row][0] === "O") markCells(board, row, 0)
    // columna derecha
    const col = board[row].length-1
    if (board[row][col] === "O") markCells(board, row, col)
  }

  for (let col=0; col < board[0].length; col++) {
    // fila arriba
    if (board[0][col] === "O") markCells(board, 0, col)
    // fila abajo
    const row = board.length-1
    if (board[row][col] === "O") markCells(board, row, col)
  }

  for (let i=0; i < board.length; i++) {
    for (let j=0; j < board[i].length; j++) {
      board[i][j] = board[i][j] === "m" ? "O" : "X"
    }
  }
};

function markCells(board, i, j) {
  board[i][j] = "m"
  if (i > 0 && board[i-1][j] === "O") {
    markCells(board, i-1, j)
  }
  if (j < board[0].length-1 && board[i][j+1] === "O") {
    markCells(board, i, j+1)
  }
  if (i < board.length-1 && board[i+1][j] === "O") {
    markCells(board, i+1, j)
  }
  if (j > 0 && board[i][j-1] === "O") {
    markCells(board, i, j-1) 
  }
}