Segmentation fault in the code

[rnk]

Im getting Segmentation fault(SIGSEGV) for many problems that iv solved in spoj and other websites. Im giving the problm statement link and code. Can anybody tell me the mistakes that iv done below.
Problem :
goo.gl/CVROl

Thanks in advance.

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
using namespace std;
int main()
{
  char a;
  string final,dummy;
  int t,h,p,k,z;
  scanf("%d",&t);
  while(t)
    {
      cin>>a>>h>>p;
      final=a;
      for(int i=0;i<h;i++)
	{
	  k=0;
	  for(int j=0;final[j]!='\0';j++)
	    {
	      if(final[j]=='a')
		{
		  dummy[k]='a';
		  dummy[k+1]='b';
		  k+=2;
		}
	      else
		{
		  dummy[k]='b';
		  dummy[k+1]='a';
		  k+=2;
		}
	    }
	  final=dummy;
	}
      printf("%c\n",final[p-1]);
      t--;
    }
  return 0;
}

[leppyr64]

You shouldn't be posting about active contests.