Leetcode no. 51

51. N-Queens

The n-queens puzzle is the problem of placing n queens on an n×n chessboard such that no two queens attack each other.

Given an integer n, return all distinct solutions to the n-queens puzzle.

Each solution contains a distinct board configuration of the n-queens' placement, where 'Q' and '.' both indicate a queen and an empty space respectively.

For example,
There exist two distinct solutions to the 4-queens puzzle:

[
 [".Q..",  // Solution 1
  "...Q",
  "Q...",
  "..Q."],

 ["..Q.",  // Solution 2
  "Q...",
  "...Q",
  ".Q.."]
]

public class Solution {
    public List<List<String>> solveNQueens(int n){
        List<List<String>> list= new LinkedList<List<String>>();
        solveNQueens(0, 0, n, new int[n], list);
        return list;
    }
    private void solveNQueens(int row, int col, int n, int[] column, List<List<String>> l){
        if (row == n){
            List<String> sub= new LinkedList<>();
            for (int j = 0; j < n; j++) {
                StringBuilder sb= new StringBuilder("");
                for (int k = 0; k < column[j]; k++) { sb.append(".");}
                sb.append("Q");
                for (int k = column[j]+1; k < n; k++) { sb.append(".");}
                sub.add(sb.toString());
            }
            l.add(sub);
        }
        else{
            for (int i = col; i < n; i++) {
                column[row]= i;
                boolean f= true;
                for (int r = row-1, lc= i-1, rc= i+1; r>=0 ; r--,lc--,rc++ ) {
                    if (column[r]== i || column[r]==lc || column[r]==rc) f= false;
                }
                if (f) solveNQueens(row+1, 0, n, column, l);
            }
        }
    }
}