var exist = function (board, word) {
  var row = board.length;
  var col = board[0].length;
  var visit = [];
  for (var x = 0; x < row; x  ) {
    visit[x] = [];
  }
  for (var i = 0; i < row; i  ) {
    for (var j = 0; j < col; j  ) {
      if (dfs(board, visit, i, j, 0, word)) {
        return true;
      }
    }
  }
  return false;
};

function dfs(board, visit, row, col, index, word) {
  if (index == word.length) {
    return true;
  }
  if (row < 0 || col < 0 || row >= board.length || col >= board[0].length) {
    return false;
  }

  var ch = word[index];
  if (!visit[row][col] && ch == board[row][col]) {
    visit[row][col] = true;
    var res = dfs(board, visit, row - 1, col, index   1, word) || dfs(board, visit, row   1, col, index   1, word) || dfs(board, visit, row, col - 1, index   1, word) || dfs(board, visit, row, col   1, index   1, word);
    visit[row][col] = false;
    return res;
  }
  return false;
}