leetcode 52. N-Queens II
The n-queens puzzle is the problem of placing n queens on an n×n chessboard such that no two queens attack each other.
Given an integer n, return the number of distinct solutions to the n-queens puzzle.
Example:
Input: 4
Output: 2
Explanation: There are two distinct solutions to the 4-queens puzzle as shown below.
[
[".Q…", // Solution 1
“…Q”,
“Q…”,
“…Q.”],
["…Q.", // Solution 2
“Q…”,
“…Q”,
“.Q…”]
]
搜索,跟上一题一样的
class Solution {
public:
int totalNQueens(int n) {
int res=0;
vector<vector<int>> a(n, vector<int>(n,0));
dfs(res, a, 0, n);
return res;
}
void dfs(int& res, vector<vector<int>>& a, int r, int n){
if(r==n){
res++;
return;
}
for(int i=0;i<n;i++){
if(!check(a,r,i,n))continue;
a[r][i]=1;
dfs(res,a,r+1,n);
a[r][i]=0;
}
}
bool check(vector<vector<int>>& a, int r, int c, int n){
for(int i=0;i<r;i++)
if(a[i][c])return false;
for(int i=r-1,j=c-1;i>=0&&j>=0;i--,j--)
if(a[i][j])return false;
for(int i=r-1,j=c+1;i>=0&&j<n;i--,j++)
if(a[i][j])return false;
return true;
}
};