Intersection of Two Linked Lists (Easy)

Description

Write a program to find the node at which the intersection of two singly linked lists begins.

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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.

Analysis

找到两个链表相交的地址。时间复杂度O(n)，空间复杂度O(1)
维护两个指针到同一长度的时候开始一起遍历

My Solution

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//C++
/**
 * 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 numa = 0,numb = 0;
        ListNode *p=headA,*q = headB;
        while(p!=NULL){
            numa++;
            p = p->next;
        }
        while(q!=NULL){
            numb++;
            q = q->next;
        }
        while(numa<numb){
            headB = headB->next;
            numb--;
        }
        while(numa>numb){
            headA = headA->next;
            numa--;
        }
        while(headA!=NULL&&headB!=NULL){
            if(headA==headB){
                return headA;
            }
            headA = headA->next;
            headB = headB->next;
        }
        return NULL;
    }
};