[LeetCode]N-Queens II

Question
Follow up for N-Queens problem.

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

本题难度Hard。

【思路】
N-Queens中返回棋盘变成了解的个数。不必多说。

【代码】

public class Solution {    int ans=0;    public int totalNQueens(int n) {        //require        if(n<=0)            return ans;        //create         int[] nqueens=new int[n];        //invariant        dfs(nqueens,n,0);        //ensure        return ans;    }    private boolean isValid(int[] nqueens,int step){        for(int i=0;i<step;i++){            if(nqueens[i]==nqueens[step]||Math.abs(nqueens[i]-nqueens[step])==(step-i))                return false;        }        return true;    }    private void dfs(int[] nqueens,int n,int step){        //base case        if(step>n-1){            ans++;            return;        }        for(int i=0;i<n;i++){            nqueens[step]=i;            if(isValid(nqueens,step))                dfs(nqueens,n,step+1);        }    }}

参考

N-Queens