相交链表

/**
 * 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;
        ListNode *p = headA;
        ListNode *q = headB;
        while(p->next != NULL){
            p = p->next;
        }
        while(q->next != NULL){
            q = q->next;
        }
        if(p != q){
            return NULL;
        }
        else{
            p = headA;
            q = headB;
            while(p != q){
                p = p->next;
                q = q->next;
                if(p == NULL)
                    p = headA;
                if(q == NULL)
                    q = headB;
            }
            return p;
        }
    }
};