谁能告诉我为什么有三个点wa呢？顺便问问 开启 O2 优化什么意思

#include<iostream>
#include<cstring>
#include<algorithm>

using namespace std;

const int N = 20;
int g[N][N];
int n;
int path[N];
int col[N], updg[N], lowdg[N];
int cnt=0;

void dfs(int u)
{
    if (u > n)
    {
        cnt ++;
        if (cnt <= 3)
        {
            for (int i=1; i <= n; i ++) //输出方案！
                cout << path[i] << " ";
            puts(""); 
        }
        return ;
    }

    for (int i=1; i <= n; i ++) //循环枚举的是列！ 
    {
        if (!col[i] && !updg[u+i] && !lowdg[n-u+i] && g[u][i]==0)
        {
            col[i] = updg[u+i] = lowdg[n-u+i] = true;
            g[u][i] = 1;
            path[u] = i;

            dfs(u+1);

            g[u][i] = 0;
            col[i] = updg[u+i] = lowdg[n-u+i] = false;
            path[u] = 0;
        }
    }
}

int main()
{
    cin >> n;
    dfs(1);
    cout << cnt;
    return 0;
}