leetcode--N-Queens II

Follow up for N-Queens problem.

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

 The idea is the same as that of Problem N-Queens problem : dfs

public class Solution {
    public int totalNQueens(int n) {
       List<Integer> solution = new ArrayList<Integer>();
		solution.add(0);
		if(n == 1) return 1;
		if(n >= 4){
			List<Integer> position = new ArrayList<Integer>();
			dfs(n, 0, position, solution);
		}
		return solution.get(0);
	}
	
	private boolean dfs(int n, int row, List<Integer> position, List<Integer> solution){
		if(row == n) return true;
		for(int i = 0; i < n; ++i){
			if(isValid(n, row * n + i, position)) {
				position.add(row * n + i);
				if(dfs(n, row + 1, position, solution))
					solution.set(0, solution.get(0) + 1);
				position.remove(row);
			}
		}
		return false;
	}
	
	private boolean isValid(int n, int k, List<Integer> position){
		for(int i = 0; i < position.size(); ++i){
			if((k % n  == position.get(i) % n) || Math.abs(k % n - position.get(i) % n) == (k / n - i))
				return false;
		}
		return true;
    }
}