【leetcode】Intersection of Two Linked Lists

 

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.

 

先从头到尾扫描，看两个链表长度相差多少。

然后让长的链表先走过这个长度差，然后两个链表同时走，直到相遇

 

/**
 * 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 *hA=headA,*hB=headB;
       
        int lenA=1,lenB=1;
        while(hA->next!=NULL)
        {
            lenA++;
            hA=hA->next;
        }
       
        while(hB->next!=NULL)
        {
            lenB++;
            hB=hB->next;
        }
       
       
        if(hA!=hB)
        {
            return NULL;
        }
       
        int dis=lenA-lenB;
       
       
        hA=headA;
        hB=headB;
       
        if(dis>0)
        {
            while(dis)
            {
                hA=hA->next;
                dis--;
            }
        }
       
        if(dis<0)
        {
            dis=-dis;
            while(dis)
            {
                hB=hB->next;
                dis--;
            }
        }         
       
        while(hA!=hB)
        {
            hA=hA->next;
            hB=hB->next;
        }
       
       
        return hA;
    }
};