#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<set>
#include <vector>
#include <string>
#include <iostream>

using namespace std;
typedef long long ll;

const int ten[4] = {1,10,100,1000};
const int maxl = 1010;
struct BigNumber
{
    
 
int d[maxl];
    BigNumber(string s)
    {
        int len = s.size();
        d[0] = (len-1)/4+1;
        int i,j,k;
        for(i=1;i<maxl;i++)
        {
            d[i] = 0;
        }
        for(i=len-1;i>=0;--i)
        {
            j=(len-i-1)/4+1;
            k=(len-i-1)%4;
            d[j]
 
+=ten[k]*(s[i]-‘0‘);
        }
        while(d[0]>1 && d[d[0]]==0) --d[0];
    }
    BigNumber()
    {
        *this=BigNumber(string("0"));
    }
    string toString()
    {
        string s("");
        int i,j,temp;
        for(i=3;i>=1;--i) if(d[d[0]]>=ten[i]) break;
        temp = d[d[0
 
]];
        for(j=i;j>=0;--j)
        {
            s = s+(char)(temp/ten[j]+‘0‘);
            temp %=ten[j];
        }
        for(i=d[0]-1;i>0;--i)
        {
            temp = d[i];
            for(j=3;j>=0;--j)
            {
                s = s+(char)(temp/ten[j]+‘0‘);
                temp %= ten[j];
            }
        }
        return s;
    }
}zero("0"),d,temp,mid1[15];

bool operator <(const BigNumber &a, const BigNumber &b)
{
    if(a.d[0]!=b.d[0]) return a.d[0]<b.d[0];
    int i;
    for(i = a.d[0];i>0;--i) if(a.d[i]!=b.d[i]) return a.d[i]<b.d[i];
    return false;
}

BigNumber operator +(const BigNumber &a,const BigNumber &b)
{
    BigNumber c;
    c.d[0] = max(a.d[0],b.d[0]);
    int i,x=0;
    for(i=1;i<=c.d[0];++i)
    {
        x = a.d[i]+b.d[i]+x;
        c.d[i] = x%10000;
        x/=10000;
    }
    while(x!=0)
    {
        c.d[++c.d[0]]=x%10000;
        x/=10000;
    }
    return c;
}

BigNumber operator -(const BigNumber &a, const BigNumber &b)
{
    BigNumber c;
    c.d[0] = a.d[0];
    int i,x=0;
    for(i=1;i<=c.d[0];++i)
    {
        x = 10000+a.d[i]-b.d[i]+x;
        c.d[i] = x%10000;
        x=x/10000-1;
    }
    while((c.d[0]>1)&&(c.d[c.d[0]]==0)) --c.d[0];
    return c;
}

BigNumber operator *(const BigNumber &a,const BigNumber &b)
{
    BigNumber c;
    c.d[0] = a.d[0]+b.d[0];
    int i,j,x;
    for(int i=1;i<=a.d[0];++i)
    {
        x = 0;
        for(j=1;j<=b.d[0];++j)
        {
            x = a.d[i]*b.d[j]+x+c.d[i+j-1];
            c.d[i+j-1] = x%10000;
            x/=10000;
        }
        c.d[i+b.d[0]] = x;
    }
    while((c.d[0]>1)&&(c.d[c.d[0]]==0)) --c.d[0];
    return c;
}

bool smaller(const BigNumber &a,const BigNumber &b,int delta)
{
    if(a.d[0]+delta!=b.d[0]) return a.d[0]+delta<b.d[0];
    int i;
    for(i=a.d[0];i>0;--i) if(a.d[i]!=b.d[i+delta])
            return a.d[i]<b.d[i+delta];
    return true;
}

void Minus(BigNumber &a,const BigNumber &b,int delta)
{
    int i,x=0;
    for(i=1;i<=a.d[0]-delta;++i)
    {
        x = 10000+a.d[i+delta]-b.d[i]+x;
        a.d[i+delta] = x%10000;
        x = x/10000-1;
    }
    while((a.d[0]>1)&&(a.d[a.d[0]]==0)) --a.d[0];
}

BigNumber operator *(const BigNumber &a,const int &k)
{
    BigNumber c;
    c.d[0] = a.d[0];
    int i,x=0;
    for(i=1;i<=a.d[0];++i)
    {
        x = a.d[i]*k+x;
        c.d[i] = x%10000;
        x/=10000;
    }
    while(x>0)
    {
        c.d[++c.d[0]] = x%10000;
        x/=10000;
    }
    while((c.d[0]>1)&&(c.d[c.d[0]]==0)) --c.d[0];
    return c;
}

BigNumber operator /(const BigNumber &a,const BigNumber &b)
{
    BigNumber c;
    d = a;
    int i,j,temp;
    mid1[0] = b;
    for(i=1;i<=13;i++)
    {
        mid1[i] = mid1[i-1]*2;
    }
    for(i=a.d[0]-b.d[0];i>=0;--i)
    {
        temp = 8192;
        for(j=13;j>=0;--j)
        {
            if(smaller(mid1[j],d,i))
            {
                Minus(d,mid1[j],i);
                c.d[i+1]+=temp;
            }
            temp /=2;
        }
    }
    c.d[0] = max(1,a.d[0]-b.d[0]+1);
    while((c.d[0]>1)&&(c.d[c.d[0]]==0)) --c.d[0];
    return c;
}

bool operator ==(const BigNumber &a,const BigNumber &b)
{
    int i;
    if(a.d[0]!=b.d[0]) return false;
    for(int i=1;i<=a.d[0];++i) if(a.d[i]!=b.d[i]) return false;
    return true;
}


string m;

int main()
{
    cin>>m;
    BigNumber M = BigNumber(m);
    BigNumber n1 = BigNumber("0");
    BigNumber n2 = BigNumber("2");
    BigNumber ans1,ans2;
    if(M==n1) ans1 = n1;
    else
    {
        BigNumber tp;
        while(n2<M)
        {
            tp = n2;
            n2 = n2*6-n1;
            n1 = tp;
        }
        ans1 = n2;
    }

    BigNumber nn1 = BigNumber("0");
    BigNumber nn2 = BigNumber("6");
    if(M==nn1) ans2 = nn1;
    else
    {
        BigNumber tp;
        while(nn2<M)
        {
            tp = nn2;
            nn2 = nn2*14-nn1;
            nn1 = tp;
        }
        ans2 = nn2;
    }

    string ans;
    if(ans1<ans2)
        ans = ans1.toString();
    else
        ans = ans2.toString();
    cout<<ans<<endl;
}

java 對應使用：

import java.math.BigInteger;
import java.util.Scanner;

public class Main {

    static Scanner in = new Scanner(System.in);
    public static void main(String[] args) {
        BigInteger m = in.nextBigInteger();
        BigInteger n0 = BigInteger.ZERO;
        BigInteger n1 = new BigInteger("2");
        while(n1.compareTo(m)<0){
            BigInteger n2 = new BigInteger("6").multiply(n1).subtract(n0);
            n0 = n1;
            n1 = n2;
        }
        BigInteger nn0 = BigInteger.ZERO;
        BigInteger nn1 = new BigInteger("6" );
        while(nn1.compareTo(m)<0){
            BigInteger nn2 = new BigInteger("14").multiply(nn1).subtract(nn0);
            nn0 = nn1;
            nn1 = nn2;
        }
        if(n1.compareTo(nn1)<0) System.out.println(n1);
        else System.out.println(nn1);
    }
}

大整數模板(+ - * / == <)