LeetCode OJ：Intersection of Two Linked Lists（两个链表的插入）

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.

找出第一个合并的节点的位置（或者说插入），注意复杂度为O(N)，那么先遍历两个链表，求出长度，把长的那个链表前面的“剪去”,下面的就好比较了，代码如下：

class Solution {
public:
    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
        int len1, len2;
        len1 = len2 = 0;
        ListNode * p = headA;
        ListNode * q = headB;
        while(p){
            p = p->next;
            len1++;
        }
        while(q){
            q = q->next;
            len2++;
        }
        if(len1 > len2){
            int diff = len1 - len2;
            while(diff--)
                headA = headA->next;
        }else if(len2 > len1){
            int diff = len2 - len1;
            while(diff--)
                headB = headB->next;
        }
        while(headA){
            if(headA == headB)
                return headA;
            headA = headA->next;
            headB = headB->next;
        }
        return NULL;
    }
};