N-Queens

Link: http://oj.leetcode.com/problems/n-queens/

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.."]
]
 1 public class Solution {
 2     
 3     public  ArrayList<String[]> result = new ArrayList<String[]>();
 4 
 5     public  ArrayList<String[]> solveNQueens(int n) {
 6         if (n < 1)
 7             return null;
 8         result = new ArrayList<String[]>();
 9         int[] queens = new int[n + 1];
10         for (int i = 1; i <= n; i++) {
11             queens[1] = i;
12             dfs(2, n, queens);
13         }
14         return result;
15     }
16 
17     public  void dfs(int row, int size, int[] queens) {
18         if (row > size) {
19             print_queen(size,queens);
20             return;
21         }
22         for (int i = 1; i <= size; i++) {
23             if (isValid(row, i, queens)) {
24                 queens[row] = i;
25                 dfs(row + 1, size, queens);
26                 queens[row] = 0;
27 
28             }
29         }
30     }
31 
32     public  boolean isValid(int row, int col, int[] queens) {
33         for (int i = 1; i < row; i++) {
34             if (queens[i] != 0
35                     && (queens[i] == col || (Math.abs(i - row) == Math
36                             .abs(queens[i] - col)))) {
37                 return false;
38             }
39         }
40 
41         return true;
42     }
43         public  void print_queen(int size,int[] queens) {
44         String[] str = new String[size];
45         for (int i = 1; i <=size; i++) {
46             int temp = queens[i];
47             String s = "";
48             for (int j = 0; j < size; j++) {
49                 if (j + 1 == temp) {
50                     s += "Q";
51                     continue;
52                 }
53                 s += ".";
54             }
55             str[i - 1] = s;
56         }
57         result.add(str);
58     }
59 }

 

 

 

posted on 2014-05-07 23:26  Atlas-Zzz  阅读(123)  评论(0)  编辑  收藏  举报

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