[leetcode] Intersection of Two Linked Lists

From : https://leetcode.com/problems/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) {        if(!headA || !headB) return NULL;        int len=0, lenb=0;        ListNode* p = headA, *q = headB;        while(p) {p=p->next; len++;}        while(q) {q=q->next; lenb++;}        if(len >= lenb) {p = headA; q = headB; len = len-lenb;}        else {p = headB; q = headA; len = lenb-len;}        while(len--) p = p->next;        while(p != q) {            p = p->next;            q = q->next;        }        return p;    }};