八皇后神级位运算解法

N-Queens

public class Solution {
    public int totalNQueens(int n) {
        upperlim = (1<<n) - 1;
        count = 0;
        dfs(0,0,0);
        return count;
    }
    
    public void dfs(int row, int leftX, int rightX) {
        if (row == upperlim) {
            count++;
            return;
        }
        
        int availables = upperlim & ~(row | leftX | rightX);
        int pos;
        
        while (availables != 0) {
            pos = availables & (-availables);
            dfs(row|pos, (leftX|pos)<<1, (rightX|pos)>>1);
            availables -= pos;
        }
    }
    
    private int upperlim;
    private int count;
}