Flipkart Interview Question Site Reliability Engineers


Team: GGn
Country: United States
Interview Type: In-Person


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7
of 7 vote

1. Take two pointers, fast and slow. At each iteration, the fast pointer moves forward two nodes and the slow pointer moves forward one node. If the two pointers meet, that means there is a loop. Break out of the loop.
2. Now start a counter to find out the length of the loop. Initialize the counter to 0. Don't move the slow pointer, only move the fast pointer forward. For every iteration, increment the counter by 1. When the slow and fast pointers meet, the counter's value will give you the length of the loop.
3. Now that you have the length of the loop (loopLength), initialize both slow and fast pointers to the head of the linked list. Start a for loop and move the fast pointer forward loopLength number of times. At this point, the slow pointer points to the head and the fast pointer is loopLength nodes ahead of the slow pointer.
4. Now just move the slow and fast pointers forward till they meet. Where they meet is the beginning of the loop.

public Node getBeginingOfLoop() {
	Node slowIterator = this.head;    // slow pointer
	Node fastIterator = this.head;    // fast pointer
	/*
	 * slow pointer moves ahead one node
	 * fast pointer moves ahead two nodes.
	 * If the two pointers meet, break out of the loop
	 */
	while(fastIterator.getNext().getNext() != null) {
		slowIterator = slowIterator.getNext();
                fastIterator = fastIterator.getNext().getNext();
		if(slowIterator == fastIterator) {
			break;
		}
	}
	/*
	 * find out the length of the loop by keeping slow
	 * pointer stationary and moving the fast pointer
	 * forward till they meet
	 */
	int loopLength = 0;
	while(slowIterator != fastIterator) {
		fastIterator = fastIterator.getNext();
		loopLength++;
	}
	slowIterator = this.head;    // initialize pointers to head
	fastIterator = this.head;
	/* move the fast pointer loopLength nodes ahead */
	for(int iii = 0; iii < loopLength; iii++) {
		fastIterator = fastIterator.getNext();
	}
	/*
	 *now the slow pointer is at the head and fast pointer is
	 * loopLength nodes ahead of slow pointer.
	 * Start moving pointers ahead till they meet
	 */
	while(slowIterator != fastIterator) {
		fastIterator = fastIterator.getNext();
		slowIterator = slowIterator.getNext();
	}
	return fastIterator;    // where they meet is the starting of the loop
}

- Bonzo on July 08, 2012 | Flag Reply
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1
of 1 vote

geeksforgeeks.org/archives/12225 code for detect and remove loop.

- Psycho on August 29, 2012 | Flag Reply
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0
of 0 vote

the diagram is not coming fine here..take it as 4 is joined to 7 ,and 0 joined to 9.

- sujita on July 07, 2012 | Flag Reply
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0
of 0 vote

circular_LL(node *root){
p=root;
q=root;
do{
p=p->next;
q=q->next->next;
}while(p->data!=q->data && p!=null);
}

This code will show the meeting point and detects the circular linked list

- Anonymous on July 07, 2012 | Flag Reply
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0
of 0 votes

no this won't do that.

- sujita on July 07, 2012 | Flag
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0
of 0 vote

Let each node contains a flag 'isVisited' which is set to 'False' initially..

while traversing the list modify the 'isVisited' variable to 'True' in each node....

when you find the node with 'isVisited' variable with 'True'.. then it is circularly linked list and this node is the meeting point.

- cobra on July 07, 2012 | Flag Reply
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0
of 0 vote

1. take two pointers fast & slow.
2. Move slow once & fast twice until they do not meet or fast becomes NULL(No cycle)
3. Move slow to root & move slow & fast one step at a time until they do not meet. This is the start of the cycle. Here is pseudo code:

findCycle(root)
{
	struct node* slow,*fast;
	slow=fast=root;
	while(fast && fast->next && fast!=slow)
	{
		fast=fast->next->next;
		slow=slow->next;
	}

	if(!fast || !(fast->next)) return NULL;

	slow=root;
	while(fast!=slow)
	{
		fast=fast->next;
		slow=slow->next;
	}
	return fast;
}

- Aashish on July 07, 2012 | Flag Reply
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0
of 0 votes

did not understand your third point here.

- sujita on July 07, 2012 | Flag
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0
of 0 votes

Fix fast at the meeting point, move slow to the root.
Now move both slow & fast once until they do not meet.

- Aashish on July 07, 2012 | Flag
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0
of 0 votes

@shondik: Even I did not understand your 3rd point here. Consider a case where cycle exists for the entire linked list i.e., 1->2->3->4->1 (the first '1') then the fast and slow pointer will never meet and the last while loop will not come out.

- CVD on July 08, 2012 | Flag
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0
of 0 votes

download cracking coding interviews pdf . this questions is explained well in it.

- chad on July 08, 2012 | Flag
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0
of 0 vote

use 2 stacks
pop out stacks when found the meeting point
O(n) space and time Complexity

- Anonymous on August 10, 2012 | Flag Reply
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-1
of 1 vote

Have two pointer which is pointed to head of the linked list initially.Then Increment the first pointer by 1, and increment the second pointer by 2 . If these 2 pointers then theres is a circle and meeting point is value to be print.

- Ani on July 07, 2012 | Flag Reply
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0
of 0 votes

but I don't think they will meet at 4 ..which we need.

- sujita on July 07, 2012 | Flag


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