LeetCode N-Queens

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 List<List<String>> solveNQueens(int n) {
//棋盘，0表示干净，n表示放有皇后n，负数表示是有多少个皇后将这个格子据为己有
int[][] chessboard = new int

;
List<List<String>> result = new ArrayList<List<String>>();
place(1,chessboard,result);
return result;
}
/*
*   将第n个皇后放到棋盘上
*/
public static void place(int n,int[][] chessboard,List<List<String>> result){
for(int j = 0; j < chessboard.length ; j++){
//格子是否干净
if(chessboard[n-1][j] == 0){
//是否是最后一个皇后
if(n == chessboard.length){
chessboard[n-1][j] = n;
result.add(printChessboard(chessboard));
chessboard[n-1][j] = 0;
//break;
}else{
//将皇后n放到棋盘格子[n-1][j]上
chessboard[n-1][j] = n;
occupy(n-1,j,chessboard);
place(n+1,chessboard,result);
chessboard[n-1][j] = 0;
clear(n-1,j,chessboard);
}
}
}

}
//占地盘
public static void occupy(int row , int col,int[][] chessboard){
int n = chessboard.length;
int i = 0;
int j = 0;
//将同一行据为己有
for(j = 0; j < n ; j ++){
if(j != col){
chessboard[row][j]--;
}
}
//将同一列据为己有
for(i = 0; i < n ; i ++){
if(i != row){
chessboard[i][col]--;
}
}
//将左上斜线据为己有
i = row - 1;
j = col - 1;
while(i>=0 && j>= 0){
chessboard[i][j]--;
i--;
j--;
}
i = row + 1;
j = col + 1;
//将右下斜线据为己有
while(i < n && j < n){
chessboard[i][j]--;
i++;
j++;
}
//将右上斜线
i = row - 1;
j = col + 1;
while(i>=0 && j < n){
chessboard[i][j]--;
i--;
j++;
}
//将左下斜线
i = row + 1;
j = col - 1;
while(i < n && j >= 0){
chessboard[i][j]--;
i++;
j--;
}
}
//清理地盘
public static void clear(int row , int col,int[][] chessboard){
int n = chessboard.length;
int i = 0;
int j = 0;
//清理同一行
for(j = 0; j < n ; j ++){
if(j != col){
chessboard[row][j]++;
}
}
//清理同一列
for(i = 0; i < n ; i ++){
if(i != row){
chessboard[i][col]++;
}
}
//清理左上斜线
i = row - 1;
j = col - 1;
while(i>=0 && j>= 0){
chessboard[i][j]++;
i--;
j--;
}
i = row + 1;
j = col + 1;
//清理右下斜线
while(i < n && j < n){
chessboard[i][j]++;
i++;
j++;
}
//将右上斜线
i = row - 1;
j = col + 1;
while(i>=0 && j < n){
chessboard[i][j]++;
i--;
j++;
}
//将左下斜线
i = row + 1;
j = col - 1;
while(i < n && j >= 0){
chessboard[i][j]++;
i++;
j--;
}
}
public static List<String> printChessboard(int[][] chessboard){
List<String> list = new ArrayList<String>();
for(int i = 0; i < chessboard.length; i++){
StringBuffer buffer = new StringBuffer();
for(int j = 0; j < chessboard[0].length; j++){
if(chessboard[i][j] > 0){
buffer.append('Q');
}else{
buffer.append('.');
}
}
list.add(buffer.toString());
}
return list;
}
}