leetcode 72: N-Queens

N-QueensMar 20 '12

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.."]
]

need improvement!

public class Solution {
    public ArrayList<String[]> solveNQueens(int n) {
        // Start typing your Java solution below
        // DO NOT write main() function
        char[][] res = new char[n][n];
        for(int i=0; i<n; i++) {
            for(int j=0;j<n;j++) {
                res[i][j] = '.';
            }
        }
        boolean[][] board = new boolean[n][n];
        ArrayList<String[]> resList = new ArrayList<String[]>();
        
        qRec(resList,res,board, 0);
        
        //convert char[][] to ArrayList<String[]>;
        return resList;
    }
    
    private void qRec(ArrayList<String[]> resList, char[][] res, boolean[][] board ,int level){
        int n = res.length;
        if( level >= n) {
            String[] ss = new String[n];
            for(int i=0;i<n;i++) {
                ss[i] = new String(res[i]);
            }
            resList.add(ss);         
            return;
        }
        
        
        for(int i=0; i<n; i++) {
            res[level][i] = 'Q';
            board[level][i] = true;
            if( check( res,board, i, level) ) {
                qRec(resList, res, board, level+1);
            }
            board[level][i] = false;
            res[level][i] = '.';
        }
        
    }
    
    private boolean check( char[][] res, boolean[][] board, int col ,int line){
        int n = res.length;
        for(int j=0; j<line; j++) {
            if( board[j][col] == true ) 
                return false;
            for(int k=0; k<n; k++) {
                if( board[j][k]==true && line-j==Math.abs(col-k))
                    return false;
            }
        }
        return true;
    }
}