Write a program to find the node at which the intersection of two singly linked lists begins.

For example, the following two linked lists:

A:          a1 → a2
                   ↘
                     c1 → c2 → c3
                   ↗            
B:     b1 → b2 → b3

begin to intersect at node c1.

Notes:

• If the two linked lists have no intersection at all, return null.
• The linked lists must retain their original structure after the function returns.
• You may assume there are no cycles anywhere in the entire linked structure.
• Your code should preferably run in O(n) time and use only O(1) memory.

Credits:
Special thanks to @stellari for adding this problem and creating all test cases.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution 
{
public:
    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) 
    {
        if(headA==NULL || headB==NULL)
			return NULL;
		//Ensure the start point isn't in the cycle;
		//Create a cycle in head B;
		ListNode* temp = headB;
		while(temp->next!=NULL)
		{
			if(temp == headA)
				return headA;
			temp = temp->next;
		}
		ListNode* tailB = temp;
		temp->next = headB;
		ListNode* slow = headA;
		ListNode* fast = headA;
		while(fast!=NULL && fast->next!=NULL)
		{
			slow = slow->next;
			fast = fast->next;
			fast = fast->next;
			if(fast==slow)
				break;
		}
		if(fast == NULL || fast->next==NULL)
		{
			tailB->next = NULL;
			return NULL;
		}
		fast = headA;
		while(fast!=slow)
		{
			fast = fast->next;
			slow = slow->next;
		}
		tailB->next = NULL;
		return slow;
    }
};

LeetCode--Intersection of Two Linked Lists