LeetCode——N-Queens II

Follow up for N-Queens problem.

Now, instead outputting board configurations, return the total number of distinct solutions.

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

题目：求有多少个独立的解决方式。

与上题一致，仅仅要每次成功后记录一下次数就可以。

package leetcode;

import java.util.ArrayList;
import java.util.List;

public class NQueensII {
	public static void main(String[] args) {
		System.out.println(new NQueensII().totalNQueens(5));
	}
	int res = 0;
    public int totalNQueens(int n) {
		List<String[]> result = new ArrayList<String[]>();
		List<Integer> cols = new ArrayList<Integer>();
		if(n <= 0)
			return 0;
		search(result,cols,n);
		return res;
    }
	public void search(List<String[]> result,List<Integer> cols,int n){
		if(cols.size() == n){
			result.add(draw(cols));
			res++;
			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";
				else
					chess[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;
	}
}