52.N-Queens II N皇后II

class Solution {
	public int sum = 0;
	public int totalNQueens(int n) {
		int[] arr = new int[n];
		Nqueen(0, n, arr);
		return sum;
	}
	public void Nqueen(int i, int n, int arr[]) {
		if (i == n) {
			sum++;
		} 
                else {
			for (int temp = 0; temp < n; temp++) {
				arr[i] = temp;
				boolean flag = true;
				for (int x = 0; x < i; x++) {
					if (arr[i] == arr[x] || (Math.abs(arr[i] - arr[x]) == Math.abs(x - i))) {
						flag = false;
						arr[i] = 0;
						break;
					}
				}
				if (flag == true) {
					Nqueen(i + 1, n, arr);
				}
			}
		}
	}
}

cclass Solution {
	int count;
	boolean bool[][];
	int n;
	public int totalNQueens(int n) {
		count = 0;
		bool = new boolean[n][n];
		this.n = n;
		dfs(0);
		return count;
	}
	private void dfs(int x) {
		if (x == n) {
			count++;
			return;
		}
		for (int i = 0; i < n; i++) {
			int temp = x;
			int flag = 0;
			while (temp >= 0) {
				if((i+flag<n&&bool[temp][i+flag])||(i-flag>=0&&bool[temp][i-flag])||bool[temp][i])break;
				temp--;
				flag++;
			}
			if(temp==-1) {
				bool[x][i] = true;
				dfs(x+1);
				bool[x][i] = false;
			}
		}
	}
}

2 * n 的原因

class Solution {
	int count;
	boolean bool_col[];
	boolean bool_x[];
	boolean bool_y[];
	int n;
	public int totalNQueens(int _n) {
		n = _n ;
		count = 0;
		bool_col = new boolean [n] ;
		bool_x = new boolean [2*n] ;
		bool_y = new boolean [2*n] ;
		dfs(0);
		return count ;
	}
	private void dfs(int i) {
		if(i == n ) {
			count++;
			return ;
		}
		for(int x = 0 ; x <n ; x++) {
			if(!bool_col[x] && !bool_x[x+i]&&!bool_y[i-x+n]) {
				bool_col[x] = bool_x[x+i] = bool_y[i-x+n] = true;
				dfs(i+1);
				bool_col[x] = bool_x[x+i] = bool_y[i-x+n] = false;

			}
		}
	}
}