51. N皇后

class Solution {
	int count;
	List ls;
	int n;
	boolean bool[][];

	public List<List<String>> solveNQueens(int n) {
		count = 0;
		ls = new LinkedList();
		bool = new boolean[n][n];
		this.n = n;
		dfs(0);
		return ls;
	}

	private void dfs(int x) {
		if (x == n) {
			ArrayList<String> path = new ArrayList<String>();
		
			for(int i = 0 ; i < n ; i++) {
				StringBuffer stb = new StringBuffer("");
				for(int j = 0 ; j < n ; j++) {
				if(bool[i][j] == true )
					stb.append("Q");
				else stb.append(".");
				}
				path.add(stb.toString());
			}
			ls.add(path);
			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;
			}
		}
	}
}