KMP - CF126B Password

for(int i = k + 1;i < lena;i++)
    {
        if(kmp[i] >= k)
        {
            if(k != 0)
            for(int i = 1;i <= k;i++)printf("%c",c[i]);
            return 0;
        }
    }

#include <bits/stdc++.h>
using namespace std;
char c[3000002];
int kmp[3000002];
int lena;
int main()
{
    scanf("%s",c + 1);
    lena = strlen(c + 1);
    int j = 0;
    for(int i = 2;i <= lena;i++)
    {
        while(j && c[j + 1] != c[i])j = kmp[j];
        if(c[j + 1] == c[i])j++;
        kmp[i] = j;
    }
    int k = kmp[lena];
    if(k == 0)//错误1：没想到kmp[lena]可能等于0，导致一开始有一个点没输出。
    {
        printf("Just a legend");
        return 0;
    }
    for(int i = k + 1;i < lena;i++)
    {
        if(kmp[i] >= k)
        {
            if(k != 0)
            for(int i = 1;i <= k;i++)printf("%c",c[i]);
            return 0;
        }
    }
    if(kmp[k] == 0 || k == 0)
    {printf("Just a legend");
        return 0;
    }
    else 
    {
        for(int i = 1;i <= kmp[k];i++)printf("%c",c[i]);
        return 0;
    }
    
    return 0;
}