N-Queens (Java)

Leetcode

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.."]
]

Slution: DFS

	/*
	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[]> result=new ArrayList<String[]> ();
	if (n<1){
	return result;
	}
	String[] rows=new String[n];
	int row=0;
	solutions(row, n, rows, result);
	return result;
	}
	// DFS soluve question
	private void solutions(int stRow, int n,String[] rows, ArrayList<String[]> result ){
	if (stRow>=n){
	result.add(rows.clone());
	return;
	}
	for(int col=0; col<n; col++){
	if (!isValid(col, stRow, rows)){
	continue;
	}
	char[] chars=new char[n];
	Arrays.fill(chars, '.');
	chars[col]='Q';
	rows[stRow]=String.copyValueOf(chars);
	solutions(stRow+1, n, rows, result);
	}
	}
	// check if current col has conflit with previous Q
	private boolean isValid(int col,int stRow, String[] rows){
	// checkColumn
	for (int i=0; i<stRow; i++){
	int qCol=rows[i].indexOf("Q");
	if (qCol==col){
	return false;
	}
	int rowDistance=Math.abs(stRow-i);
	int colDistance=Math.abs(col-qCol);
	if (rowDistance==colDistance){
	return false;
	}
	}
	return true;
	}
	}

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