LeetCode--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.."]]

原题链接：https://oj.leetcode.com/problems/n-queens/

题目：n*n的棋盘上放n个皇后，使得任何两个皇后不互相攻击。

思路：递归+回溯。

import java.util.ArrayList;import java.util.List;public class NQueens {public static void main(String[] args) {List<String[]> result = new NQueens().solveNQueens(5);for(String[] li:result){for(String str : li){System.out.println(str);}System.out.println("----------------");}}public List<String[]> solveNQueens(int n) {List<String[]> result = new ArrayList<String[]>();List<Integer> cols = new ArrayList<Integer>();if(n <= 0)return result;search(result,cols,n);return result;}public void search(List<String[]> result,List<Integer> cols,int n){if(cols.size() == n){result.add(draw(cols));return;}for(int col=0;col<n;col++){if(!isValid(cols,col))continue;cols.add(col);search(result,cols,n);cols.remove(cols.size()-1);}}public String[] draw(List<Integer> cols){String[] chess = new String[cols.size()];for(int i=0;i<chess.length;i++){chess[i] = "";for(int j=0;j<cols.size();j++){if(j==cols.get(i))chess[i] += "Q";elsechess[i] += ".";}}return chess;}public boolean isValid(List<Integer> cols,int col){int row = cols.size();for(int i=0;i<row;i++){if(cols.get(i) == col)return false;if(i - cols.get(i) == row - col)return false;if(i+cols.get(i) == row+col)return false;}return true;}}

reference:http://www.snip2code.com/Snippet/45053/N-Queens--LeetCode

http://huntfor.iteye.com/blog/2039013