[Leetcode]160. Intersection of Two Linked Lists

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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) {if (headA == NULL || headB == NULL)return NULL;int i = 1;int j = 1;ListNode* p = headA;ListNode* q = headB;while (p->next != NULL){i++;p = p->next;}while (q->next != NULL){j++;q = q->next;}if (p != q)return NULL;if (i >= j){for (int k = 0; k != i-j; ++k)headA = headA->next;}else{for (int k = 0; k != j-i; ++k)headB = headB->next;}while (headA != headB){headA = headA->next;headB = headB->next;}return headA;}
};

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