[LeetCode] N-Queens

Total Accepted: 7379 Total Submissions: 29099

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 ArrayList<String[]> solveNQueens(int n) {        ArrayList<String[]> list = new ArrayList<String[]>();        int[]               path = new int[n];                dfs(list, path, n, 0);                return list;    }        public void dfs(ArrayList<String[]> list, int[] path, int n, int level) {        if (level == n) {            String[] set = new String[n];                        for (int i = 0; i < n; i++) {                StringBuffer str = new StringBuffer();                for (int j = 0; j < n; j++) {                    if (path[i] == j)   str.append("Q");                    else                str.append(".");                }                set[i] = str.toString();            }                        list.add(set);            return;        }                for (int i = 0; i < n; i++) {            path[level] = i;            if (check(path, level)) dfs(list, path, n, level + 1);        }    }        public boolean check(int[] path, int level) {        for (int i = 0; i < level; i++) {            if ((path[i] == path[level]) || Math.abs(path[i] - path[level]) == (level - i)) return false;        }                return true;    }}