【leetcode刷题笔记】N-Queens II

Follow up for N-Queens problem.

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

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题解：参见http://www.cnblogs.com/sunshineatnoon/p/3853170.html

只要求求出解的个数，稍微改一下代码就可以了：用私有变量result记录解的数目，当搜索到一个解的时候，result+1即可。

代码如下：

 public class Solution {
 private int result = 0;
     private boolean isValid(List<Integer> cols,int col){
         for(int i = 0;i < cols.size();i++){
             if(cols.get(i) == col)
                 return false;
             if(cols.size() - i == Math.abs(cols.get(i) - col))
                 return false;
         }
         return true;
     }
     private void QueenDfs(int n,int level,List<Integer> cols){
         if(level == n){
             result++;
         }

         for(int i = 0;i < n;i++){
             if(isValid(cols, i)){
                 cols.add(i);
                 QueenDfs(n, level+1, cols);
                 cols.remove(cols.size()-1);
             }
         }
     }
     public int totalNQueens(int n) {

         List<Integer> cols = new ArrayList<Integer>();
         QueenDfs(n, 0, cols );

         return result;
     }
 }