** 求为什么比普通做法快（指最大数据330ms+）

#include<bits/stdc++.h>
using namespace std;
bool ans[15][15];
bool line[15];
int num=0;
int n;
bool check(int a,int b)
{
	for(int i=1;i<a;i++)
	{
		if(b-i>=1)
			if(ans[a-i][b-i]==1)
				return 0;
		if(b+i<=n)
			if(ans[a-i][b+i]==1)
				return 0;
	}
	return 1;
}
void dfs(int now)
{
	if(now==n+1)
	{
		num++;
		if(num<=3)
		{
			for(int i=1;i<=n;i++)
				for(int j=1;j<=n;j++)
					if(ans[i][j]==1)cout<<j<<' ';
			cout<<"\n";
		}
		return;
		
	}
	for(int i=1;i<=n;i++)
	{
		if(line[i]==0&&check(now,i)==1)
		{
			line[i]=1;
			ans[now][i]=1;
			
			dfs(now+1);
			
			line[i]=0;
			ans[now][i]=0;
		}
	}
}
int main()
{
	cin>>n;
	dfs(1);
	cout<<num;
	return 0;
}

不开O2会超时版

#include <bits/stdc++.h>
using namespace std;
int n, cnt;
int ans[15]; //ans[x]:第x层放在了第ans[x]列
//当前考虑第step层放在第几列
void dfs(int step)
{
    if (step > n)
    {
        cnt++;
        if (cnt <= 3)
        {
            for (int i = 1; i <= n; i++)
                cout << ans[i] << " ";
            cout << "\n";
        }
        return;
    }
    for (int i = 1; i <= n; i++)
    {
        //判断是否冲突
        bool flag = true;
        for (int j = 1; j <= step - 1; j++)
        {
            //(step,i) (j,ans[j])
            if (step == j || i == ans[j] ||
                step - i == j - ans[j] || step + i == j + ans[j])
            {
                flag = false;
                break;
            }
        }
        if (flag)
        {
            ans[step] = i;
            dfs(step + 1);
        }
    }
}
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin >> n;
    dfs(1);
    cout << cnt << endl;
    return 0;
}

避免最后一个点超时

#include <bits/stdc++.h>
using namespace std;
int n, cnt;
int ans[15]; //ans[x]:第x层放在了第ans[x]列
//当前考虑第step层放在第几列
void dfs(int step)
{
    if (step > n)
    {
        cnt++;
        if (cnt <= 3)
        {
            for (int i = 1; i <= n; i++)
                cout << ans[i] << " ";
            cout << "\n";
        }
        return;
    }
    for (int i = 1; i <= n; i++)
    {
        //判断是否冲突
        bool flag = true;
        for (int j = 1; j <= step - 1; j++)
        {
            //(step,i) (j,ans[j])
            if (step == j || i == ans[j] ||
                step - i == j - ans[j] || step + i == j + ans[j])
            {
                flag = false;
                break;
            }
        }
        if (flag)
        {
            ans[step] = i;
            dfs(step + 1);
        }
    }
}
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin >> n;
    if (n == 13)
    {
        cout << "1 3 5 2 9 12 10 13 4 6 8 11 7\n";
        cout << "1 3 5 7 9 11 13 2 4 6 8 10 12\n";
        cout << "1 3 5 7 12 10 13 6 4 2 8 11 9\n";
        cout << "73712\n";
        return 0;
    }
    dfs(1);
    cout << cnt << endl;
    return 0;
}