How to compare and substitute strings in different lines in unix

Question

I want to compare and substitute strings present in different lines in unix

For example I have a file with two words in each line

Answer 1

awk '{i++;a[i]=$1;b[i]=$2;next}
      END{
            for(i=1;i in a;i++)
            {
              f=1;
              while (f==1)
              {
                f=0;
                for(j=i+1;j in a;j++)
                {
                  if(b[i]==a[j])
                  {
                    b[i]=b[j];
                    f=1;
                  }
                }
              }
            }
            for(i=1;i in a;i++)
            {
              print a[i],b[i];
            }
          }' input.txt

Input:

Output:

Input:

Output:

---

EDIT

If you need to get

As output from the second input you can change this line:

if(b[i]==a[j])

to:

if(j!=i&&b[i]==a[j])

and this:

for(j=i+1;j in a;j++)

to:

for(j=1;j in a;j++)

Also note that this code assumes there is not a case where second word of a line is equal to both first word of a line and its second word i.e:

In that case the execution of the code will never ends.