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备注：
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#include <queue>
#include <math.h>
#include <stack>
#include <stdio.h>
#include <iostream>
#include <vector>
#include <iomanip>
#include <string.h>
#include <algorithm>
using namespace std;
#define LL long long
const int N = 1e5 + 10;
const int INF = 0x3f3f3f3f;
int main()
{
	string a;
	cin >> a;
	int len = a.size();
	while(len > 1)
	{
		int sum =0;
		for(int i = 0 ; i < len ; i++)
			sum += a[i] - 48;
		a = to_string(sum);
		len = a.size();
	}
	cout << a << endl;
	return 0;
}

#include<bits/stdc++.h> using namespace std; #define LL long long const int N = 1e5 + 10; const int INF = 0x3f3f3f3f; int main() { string a; cin >> a; int len = a.size(); while(len > 1) { int sum =0; for(int i = 0 ; i < len ; i++) sum += a[i] - 48; a = to_string(sum); len = a.size(); } cout << a << endl; return 0; }

#include<bits/stdc++.h> using namespace std; #define LL long long const int N = 1e5 + 10; const int INF = 0x3f3f3f3f; int main() { string a; cin >> a; int len = a.size(); while(len > 1) { int sum =0; for(int i = 0 ; i < len ; i++) sum += a[i] - 48; a = to_string(sum); len = a.size(); } cout << a << endl; return 0; }

#include<bits/stdc++.h> using namespace std; #define LL long long const int N = 1e5 + 10; const int INF = 0x3f3f3f3f; int main() { string a; cin >> a; int len = a.size(); while(len > 1) { int sum =0; for(int i = 0 ; i < len ; i++) sum += a[i] - 48; a = to_string(sum); len = a.size(); } cout << a << endl; return 0; }

#include<bits/stdc++.h> using namespace std; #define LL long long const int N = 1e5 + 10; const int INF = 0x3f3f3f3f; int main() { string a; cin >> a; int len = a.size(); while(len > 1) { int sum =0; for(int i = 0 ; i < len ; i++) sum += a[i] - 48; a = to_string(sum); len = a.size(); } cout << a << endl; return 0; }

递归

#include<bits/stdc++.h>
using namespace std;
const int N=1e2+5,INF=0x3f3f3f3f;
string a;
int len,x;
int f(int x){
	int sum=0;
	if(x/10==0)return x;
	else{
		while(x){
			sum+=x%10;
			x/=10;
		}
		return f(sum);
	}
}
int main()
{
	cin>>a;
	len = a.size();
	for(int i=0;i<len;i++)x+=a[i]-48;
	cout<<f(x);
	return 0;
}

#include <queue> 
#include <math.h> 
#include <stack> 
#include <stdio.h>
#include <iostream>
#include <vector> 
#include <iomanip> 
#include <string.h> 
#include <algorithm>
using namespace std;
#define LL long long
const int N = 1e5 + 10; 
const int INF = 0x3f3f3f3f; 
char a[N];
int main()
{
	cin >> a;
	int len = strlen(a);
	int sum = 0;
	for (int i = 0 ; i < len; i++)
	{
		sum +=  (a[i] - '0');
	}
	while(sum >= 10)
	{
		int ans = 0;
		while(sum != 0)
		{
			ans += sum%10;
			sum /= 10;

		}
		sum = ans;
	}
	cout << sum << endl;


}

#include <queue> 
#include <math.h> 
#include <stack> 
#include <stdio.h>
#include <iostream>
#include <vector> 
#include <iomanip> 
#include <string.h> 
#include <algorithm>
using namespace std;
#define LL long long
const int N = 1e5 + 10; 
const int INF = 0x3f3f3f3f; 
char a[N];
int main()
{
	cin >> a;
	int len = strlen(a);
	int sum = 0;
	for (int i = 0 ; i < len; i++)
	{
		sum +=  (a[i] - '0');
	}
	while(sum >= 10)
	{
		int ans = 0;
		while(sum != 0)
		{
			ans += sum%10;
			sum /= 10;

		}
		sum = ans;
	}
	cout << sum << endl;


}

#include <string.h>
 
#define M 1000000
 
int main(void)
{
    long long a, b;
    char s[M+1];
    int i;
 
    while (scanf("%s", s) != EOF)
    {
        if (strcmp(s, "0") == 0)
            break;
 
        a = 0;
        for (i=0; s[i]; i++)
            a += s[i]-'0';
 
        while (a>=10)
        {
            b = 0;
            while (a)
            {
                b += a%10;
                a /= 10;
            }
            a = b;
        }
 
        printf("%lld\n", a);
    }
 
    return 0;
}```

#include <bits/stdc++.h> using namespace std; int n; int sum; int g; int main(){ cin >> n; while(n != 0){ g = n % 10; n /= 10; sum += g; } cout << sum << endl; return 0; } 20分题解！！！

#include #include <stdio.h> #include <string.h> using namespace std; int main() { int n; cin >> n; int sum = 0; while(1) { if(n == 0) { if(sum >= 10) { n = sum; sum = 0; } else { cout << sum; break; } } else { sum += n%10; n = n/10; } } }