leetcode51. N 皇后（java详解）

class Solution {
    

    private List> ans = new ArrayList<>();
    private char[][] arr;
    
    public void fillArr() {
        for (char[] chA: arr) {
            Arrays.fill(chA,'.');
        }
    }

    public List getString() {
        List list = new ArrayList<>();
        for (char[] chA: arr) {//将二维char数组中的每一行转换成一个字符串 存放在list中
            list.add(String.copyValueOf(chA));
        }
        return list;
    }

    //判断坐标(row,col)这个坐标是否合法
    public boolean isLegal(int row,int col,int n) {    
        //是否在同一列
        for (int i = 0; i < row; i++) {
            if (arr[i][col] == 'Q') return false;
        }

        //45°角
        for (int i = row-1,j = col-1; i >= 0 && j >= 0; i--,j--) {
            if (arr[i][j] == 'Q') return false;
        }

        //135°
        for (int i = row-1,j = col+1; i >= 0 && j < n; i--,j++) {
            if (arr[i][j] == 'Q') return false;
        }

        return true;
    }
    
    public void getAns(int rows,int n) {
        
        if (rows == n) {
            ans.add(getString());
        }

        for (int col = 0; col < n; col++) {
            if (isLegal(rows,col,n)) {
                arr[rows][col] = 'Q';
                getAns(rows+1,n);
                arr[rows][col] = '.';
            }
        }
    }
    public List> solveNQueens(int n) {
        arr = new char[n][n];
        fillArr();
        getAns(0,n);
        return ans; 
    }
}