【Leetcode】N-Queens (Backtracking)

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.."]]

N皇后问题的基本思路就是回朔法，往每一个格子放棋子，如果是合法的就继续，如果不合法就撤掉

用一维数组解这种题的时候，可以忽略掉撤掉的情况，因为当用一维数组的时候，只存在放与不放的问题，不存在放与撤掉的问题

代码如下

来自Code_Ganker

public ArrayList<String[]> solveNQueens(int n) {ArrayList<String[]> result = new ArrayList<String[]>();if (n <= 0)return result;int[] column = new int[n];helper(n, 0, column, result);return result;}public void helper(int n, int row, int[] column, ArrayList<String[]> result) {if (n == row) {String[] temp = new String[n];for (int i = 0; i < n; i++) {StringBuilder strRow = new StringBuilder();for (int j = 0; j < n; j++) {if (column[i] == j)strRow.append("Q");elsestrRow.append(".");}temp[i] = strRow.toString();}result.add(temp);return;}for (int i = 0; i < n; i++) {column[row] = i;if (check(row, column))helper(n, row + 1, column, result);}}public boolean check(int row, int[] column) {for (int i = 0; i < row; i++) {if (column[i] == column[row]|| Math.abs(column[row] - column[i]) == (row - i))return false;}return true;}