这题和LeetCode第51题做法一样，只不过不是记录具体方案，而是个数。分析见第51题。
代码如下：
class Solution {
int res = 0;
int n;
vector<string> path;
vector<bool> cols, diagram, anti_diagram;
public:
int totalNQueens(int _n) {
n = _n;
path = vector<string>(n, string(n, \'.\'));
cols = vector<bool>(n);
diagram = anti_diagram = vector<bool>(2 * n);
DFS(0);
return res;
}
void DFS(int curRow) {
if(curRow == n) {
++res;
return ;
}
for(int i = 0; i < n; ++i) {
if(cols[i] == false && diagram[curRow - i + n] == false && anti_diagram[curRow + i] == false) {
cols[i] = diagram[curRow - i + n] = anti_diagram[curRow + i] = true;
path[curRow][i] = \'Q\';
DFS(curRow + 1);
path[curRow][i] = \'.\';
cols[i] = diagram[curRow - i + n] = anti_diagram[curRow + i] = false;
}
}
}
};