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Yahoo Software Engineer Interview Question

"How to find out if a linked list has a loop?"
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The hare and tortoise algorithm can be used. Start at node X and X+1. Have 2 pointers. One goes to next() and another goes to next().next() [twice as fast]. At some point, both pointers will be on same node Y. If you reach a null for second pointer, at any point, then there is no cycle.
- Anonymous on Nov 6, 2013 Flag Response
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boolean hasLoopInList(Node first){
  if(first == null){
   return false;

Node slow,first ;

  slow = first = first ;


  slow = ;
if( !=null){
  fast =;

  if(first ==null || slow ==null) return false;

 if(slow == fast) // if the two ever meet...we must have a loop.
            return true;


- Soumya on Nov 7, 2013 Flag Response

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