High level：

public class Solution {
  public List<List<Integer>> nqueens(int n) {
    List<List<Integer>> result = new ArrayList<>();
    List<Integer> cur = new ArrayList<>();
    helper(result, cur, n);
    return result;
  }

  private void helper(List<List<Integer>> result, List<Integer> cur, int n) {
    // base case
    if (cur.size() == n) {
      result.add(new ArrayList<Integer>(cur));
      return;
    }
    for (int i = 0; i < n; i++) {
      if (isValid(i, cur)) {
        cur.add(i);
        helper(result, cur, n);
        cur.remove(cur.size() - 1);
      }
    }
  }

  private boolean isValid(int column, List<Integer> cur) {
    int row = cur.size();
    for (int i = 0; i < row; i++) {
      if (cur.get(i) == column) {
        return false;
      } 
      if (Math.abs(cur.get(i) - column) == row - i) {
        return false;
      }
    }
    return true;
  }
}