leetcode-N-Queens 搜索

问题

题目：[leetcode-51]

思路

dfs搜索即可，注意剪枝的条件。
注意，此时的搜索要恢复状态。

代码

class Solution {public:    vector<vector<string>> solveNQueens(int n) {        vector<vector<string>> ret;        if( n < 1 ) return ret;        vector<int> visited;        dfs(n, 0, visited, ret);        return ret;    }private:    void dfs(int n, int cur, vector<int>& visited, vector<vector<string>>& ans){        if( cur == n ){            vector<string> solution;            for(int i = 0; i < n; ++i){                int loc = visited[i];                string s;                for(int j = 0; j < n; ++j){                    if( j == loc ) s.push_back('Q');                    else s.push_back('.');                }                solution.push_back(s);            }            ans.push_back( solution );        }        else{            for(int k = 0; k < n; ++k){                if(valid( cur, k, visited )){                    visited.push_back( k );                    dfs( n, cur+1, visited, ans );                    visited.pop_back();                }            }        }    }// dfs    bool valid( int x, int y, vector<int>& visited ){        int sz = visited.size();        for( int u = 0; u < sz; ++u ){            int v = visited[u];            // same column            if( y == v ) return false;            // same diag            if( abs(x-u) == abs(y-v) ) return false;        }        return true;    }// valid};