n-queens.js

/**
 * @param {number} n
 * @return {string[][]}
 */

var solveNQueens = function(n) {

    var res = [];
    if (n === 1 || n >= 4) 
        dfs(res, [], n, 0);
    return res;
};

var dfs = function (res, points, n, index) {
  for (var i = index; i < n; i++) {
    if (points.length !== i) 
        return;
    for (var j = 0; j < n; j++) {
      if (isValid(points, [i, j])) {
        points.push([i, j]);
        dfs(res, points, n, i + 1);
        if (points.length === n) 
            res.push(buildRes(points));
        points.pop();
      }
    }
  }
};

var buildRes = function (points) {
  var res = [];
  var n = points.length;
  for (var i = 0; i < n; i++) {
    res[i] = '';
    for (var j = 0; j < n; j++) {
      res[i] += (points[i][1] === j ? 'Q' : '.');
    }
  }
  return res;
};

var isValid = function (oldPoints, newPoint) {
  var len = oldPoints.length;
  for (var i = 0; i < len; i++) {
    if (oldPoints[i][0] === newPoint[0] || oldPoints[i][1] === newPoint[1]) 
        return false;
    if (Math.abs((oldPoints[i][0] - newPoint[0]) / (oldPoints[i][1] - newPoint[1])) === 1) 
        return false;
  }
    
  return true;
    
};