N-Queens.java

/*
 Author:     King, nkuwjg@gmail.com
 Date:       Jul 25, 2013
 Problem:    N-Queens
 Difficulty: Medium
 Source:     https://oj.leetcode.com/problems/n-queens/
 Notes:
 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.."]
 ]

 Solution: Recursion (DFS). Use bit-manipulation solution (See N-QueensII for more details).
 */


public class Solution {
    public List<String[]> solveNQueens(int n) {
        List<String[]> res = new ArrayList<String[]>();
        List<char[]> sol = new ArrayList<char[]>();
        solveNQueensRe(n, 0, 0, 0, sol, res);
        return res;
    }
    public void solveNQueensRe(int n, int row, int ld, int rd, List<char[]> sol, List<String[]> res) {
        if (row == (1<<n) -1 ) {
            String[] temp = new String[n];
            for (int i = 0; i < n; ++i) 
                temp[i] = String.valueOf(sol.get(i)); 
            res.add(temp);
            return;
        }
        int avail = ~(row | ld | rd);
        for (int i = n -1; i >= 0; --i) {
            int pos = 1 << i;
            if ((int)(avail & pos) != 0) {
                char[] str = new char[n];
                Arrays.fill(str, '.');
                str[i] = 'Q';
                sol.add(str);
                solveNQueensRe(n, row | pos, (ld|pos)<<1, (rd|pos)>>1, sol, res);
                sol.remove(sol.size()-1);
            }
        }
    }  
}