Interview Question for Software Development Engineer (SDE) at Microsoft:

Nov 20, 2012

Answer : If he means a boolean function to see if there is a loop in the linked list or not, then the answer is to use two pointers (slow and fast). "slow" walks one step every time and "fast" walks two step every time , if you reach null with fast pointer , then there is no loop , else if the fast and slaw pointer meet then there is a loop.

if he means find the begin node of this loop (loop node), then you will write some extra code.
you now get the node where "fast" and "slow" pointers meet in. then, arithmetically, the distance between that node and the "loop node" is equal to the distance between the head of the linked list and the "loop node". what we can do is after the slaw and fast pointers meet , fast pointer will point to the head of the linked list and both the slaw and the fast pointers will continue with speed equal to one step every time until they meet. the "loop node" is the node they met in.