水题。 AC代码：

#include<iostream>
using namespace std;
int n;
bool check(int x){
	int y=x*x;
	while(x!=0){
		if(x%10!=y%10)return false;
		x/=10;
		y/=10;
	}
	return true;
}
int main(){
	cin>>n;
	for(int j=2;j<=n;j++){
		if(check(j))cout<<j<<" ";
	}
	return 0;
}

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#include<iostream>
#include<iomanip>
#include<math.h>
using namespace std;
int main(){
	int n;
	cin>>n;
	for(int i=2;i<=n;i++){
		if(i==i*i%10||i==i*i%100||i==i*i%1000){
			cout<<i<<" ";
		}
	}
}

#include <bits/stdc++.h>
using namespace std;
const int N=32000+10;
const int INF=0x3f3f3f3f;
int r;
bool c(int x)
{
	long long y=x*x;
	long long sum=1;
	while(x>=10*sum)
	{
		sum*=10;
	}
	if( (y-x) % (10*sum) ==0)
	{
		return 1;
	}
	return 0;
}
int main()
{
	cin>>r;
	for(int i=2;i<=r;i++)
	{
		if(c(i))
		{
			cout<<i<<" ";
		}
	}
	return 0;
}

水

#include <iostream>
#include <stack>
#include <cmath>
#include <vector>
#include <string.h>
#include <queue>
#include <stdio.h>
#include <iomanip>
#include <cstdio>
#include <algorithm>
#define int long long
using namespace std;
const int N = 1e5 + 10;
const int INF = 0x3f3f3f3f;
int wei(int x)
{
	int num = 1;
	while(x >= 10)
	{
		num ++;
		x /= 10;
	}
	return num;
}
bool jian(int x, int y)
{
	int sum = x - y;
	int len = wei(y);
	if(len == 1)
	{
		if(sum % 10 == 0)
		{
			return 1;
		}
	}
	if(len == 2)
	{
		if(sum % 100 == 0)
		{
			return 1;
		}
	}
	if(len == 3)
	{
		if(sum % 1000 == 0)
		{
			return 1;
		}
	}
	else
	{
		if(sum % 10000 == 0)
		{
			return 1;
		}
	}
	return 0;
}
int fang(int x)
{
	return x * x;
}
signed main()
{
	int n;
	cin >> n;
	for(int i = 2; i <= n; i++)
	{
		if(jian(fang(i), i) == 1)
		{
			cout << i << " ";
		}
	}
	cout << endl;
	return 0;
}

我太菜了

//***************************

//dev c++[[990.cpp]]*******

//dev c++[top fanil]]******

//dev c++*www.temege.com

#include <stdio.h>

#include <iostream>

#include <math.h>

#include <queue>

#include <stack>

#include <vector>

#include <iomanip>

using namespace std;

int main()

{

int n;

cin>>n;

for(int i=2;i<=n;i++)

{

	int k=i*i;

	if(i<10)

	{

		if(k%10==i)

		{

			cout<<i<<" ";

		}

	}

	else if(i<100)

	{

		if(k%100==i)

		{

			cout<<i<<" ";

		}

	}

	else

	{

		if(k%1000==i)

		{

			cout<<i<<" ";

		}

	}

}

return 0;

}

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#include <iostream> #include <math.h> using namespace std; int main() {#include <iostream> #include <math.h> using namespace std; int main() { int n; cin >> n; for (int i = 5; i <= n; i++) { int sum = ii; int flag = 0; int ans = 0; for (int j = 1; sum!= 0;j = j10) { ans = ans +(sum%10)*j; sum/=10; if (ans == i) { flag = 1; }

    }
    if (flag == 1)
    {
        cout<< i << " ";
    }
}

}

#include <iostream>
#include <math.h>
using namespace std;
int main()
{
    int n;
    cin >> n;
    for (int i = 5; i <= n; i++)
    {
        int sum = i*i;
        int flag = 0;
        int ans = 0;
        for (int j = 1; sum!= 0;j = j*10)
        {
            ans = ans +(sum%10)*j;
            sum/=10;
            if (ans == i)
            {
                flag = 1;
            }
        
        
        }
        if (flag == 1)
        {
            cout<< i << " ";
        }
    }
    
}

#include <stdio.h>
#include <iostream>
#include <math.h>
using namespace std;
int main()
{
	int n;
	cin>>n;
	for(int i=5;i<=n;i++)
	{
		int sum=i*i;
		int flag=0;
		int ans=0;
		for(int j=1;sum!=0;j=j*10)
		{
			ans=ans+(sum%10)*j;
			sum/=10;
			if(ans==i)
			{
				flag=1;
			}
		}
		if(flag==1)
		{
			cout<<i<<" ";
		}
	}
}

#include <iostream>
#include <math.h>
using namespace std;
int main()
{
    int n;
    cin >> n;
    for (int i = 5; i <= n; i++)
    {
        int sum = i*i;
        int flag = 0;
        int ans = 0;
        for (int j = 1; sum!= 0;j = j*10)
        {
            ans = ans +(sum%10)*j;
            sum/=10;
            if (ans == i)
            {
                flag = 1;
            }
        
        
        }
        if (flag == 1)
        {
            cout<< i << " ";
        }
    }
    
}

#include <iostream>
#include <math.h>
using namespace std;
int main()
{
    int n;
    cin >> n;
    for (int i = 5; i <= n; i++)
    {
        int sum = i*i;
        int flag = 0;
        int ans = 0;
        for (int j = 1; sum!= 0;j = j*10)
        {
            ans = ans +(sum%10)*j;
            sum/=10;
            if (ans == i)
            {
                flag = 1;
            }
        
        
        }
        if (flag == 1)
        {
            cout<< i << " ";
        }
    }
    
}