最多平方两次，个位就开始重复，所以也可以用循环。

import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);

        long x=sc.nextLong();
        long m= sc.nextLong();
        x=x%10;
        for(int i=0;i<2&&i<m;i++){//防止m=1情况
            x=(x*x)%10;//求个位
        }
        System.out.println(m*x);
    }
}

打表可知最多平方三次，后面就开始重复

#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define endl '\n'
void solve();

int main(){
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	int t=1;
//	cin>>t;
	while(t--){
		solve();
	}
	return 0;
}

void solve(){
	ll x,m;
	cin>>x>>m;
	for(int i=1;i<min(m,4ll);i++){
		x*=x;
		x%=10;
	}
	cout<<x*m<<endl;
}

蒟蒻写法 记得开long

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int x = sc.nextInt();
        int m = sc.nextInt();
        x %= 10;
        if(x == 1){
            System.out.println((long)m);
        }else if(x == 2){
            if(m >= 3){
                System.out.println((long)m * 6);
            }else {
                for (int i = 0; i < m - 1; i++) {
                    x *= x;
                }
                x %= 10;
                System.out.println((long)x * m);
            }
        } else if (x == 3) {
            if(m >= 3){
                System.out.println((long)m);
            }else {
                for (int i = 0; i < m - 1; i++) {
                    x *= x;
                }
                x %= 10;
                System.out.println((long)x * m);
            }
        } else if(x == 4){
            if(m >= 2){
                System.out.println((long)m * 6);
            }else System.out.println((long)4 * m);
        }else if(x == 5){
            System.out.println((long)5 * m);
        }else if(x == 6){
            System.out.println((long) 6 * m);
        }else if(x == 7){
            if(m >= 3){
                System.out.println((long)m);
            }else {
                for (int i = 0; i < m - 1; i++) {
                    x *= x;
                }
                x %= 10;
                System.out.println((long)x * m);
            }
        }else if(x == 8){
            if(m >= 3){
                System.out.println((long)6 * m);
            }else {
                for (int i = 0; i < m - 1; i++) {
                    x *= x;
                }
                x %= 10;
                System.out.println((long)x * m);
            }
        }else if(x == 9){
            if(m >= 2){
                System.out.println((long)m);
            }else {
                System.out.println((long)9 * m);
            }
        }else System.out.println(0);
    }
}

打表发现当m>一定值时，x不变化

#include <iostream>
#include <vector>
#include <set>
using namespace std;
#define int long long
const int mod=1e+7;
signed main() {
    iostream::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
   int x,m;cin>>x>>m;
   x%=10;
   int n=m;
   while((x*x)%10!=x&&m>0){
    x=(x*x)%10;
    m--;
   }
   cout<<x*n<<endl;
    return 0;//AC
 }