Leetcode45：Intersection of Two Linked Lists

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.

/**
 * 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) {
        int lenA = 0;
	int lenB = 0;
	ListNode *head1 = headA;
<span style="white-space:pre">	</span>ListNode *head2 = headB;
	while (head1!=NULL)
	{
		head1 = head1->next;
		lenA++;
	}
	while (head2!=NULL)
	{
		head2 = head2->next;
		lenB++;
	}
	int n;
	if (lenA < lenB)
	{
		n = lenB - lenA;
		head1 = headA;
		head2 = headB;
		for (int i = 0; i < n; i++)
		{
			head2 = head2->next;
		}
	}
	else
	{
		n = lenA - lenB;
		head1 = headA;
		head2 = headB;
		for (int i = 0; i < n; i++)
		{
			head1 = head1->next;
		}
	}

	while (head1 != head2)
	{
		head1 = head1->next;
		head2 = head2->next;
	}

	return head1;
    }
};

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