上传输入时出现SIGSEGV错误

我试图在ideone中解决大海捞针问题，但我得到了一个SIGSEGV。

这是我的代码:

    //start
    #include<cstring>
    #include<cstdio>
    #include<vector> 
    using namespace std;

    int *overlap;
    char *pattern;
   //used Knuth morris prat algo
   void calcoverlap()
   {
    overlap[0] = 0;
    unsigned int length,i,len;  
    length=strlen(pattern);
    while(i<length)
    {
        if (pattern[i] == pattern[len])
        {
            len++;
            overlap[i] = len;
            i++;
        }
        else
        {
            if (len != 0)
            {
                len = overlap[len-1];
            }
            else
            {
                overlap[i++] = 0;
            }
        }
    }
   }
    //this is final function for pattern matching
    vector< int > patternmatching(int m)
{
vector< int > V;
int i = 0, j = 0;
char ch;
while(1)
{
ch = getchar();
if(ch == 'n') break;
while(1)
{
if(ch == pattern[j])
{
j++;
if(j == m)
{
V.push_back(i-m+1);
j = overlap[j];
}
break;
}
else if(j == 0) break;
else j = overlap[j];
}
i++;
}
    return V;
    }


   int main()
   {
        int n,i,sz;
        vector<int> V;
        while(scanf("%d",&n)==1)
        {   
            gets(pattern);
            calcoverlap();
            V=patternmatching(n);
            sz = V.size();
           for(i=0; i < sz; i++)
            printf("%dn",V[i]);
            if(!sz) printf("n");
            delete[] pattern;
            delete[] overlap;
        }
         return 0;
        }    

有人可以解释为什么我得到这个错误只有在上传输入时，正常情况下，这个程序运行得很好和漂亮。