leetcode 160. Intersection of Two Linked Lists

160. 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.

计算长度法。假设链表A和B相交，则交点及之后的部分长度是相同的，相差的是交点前的部分。计算A和B的长度差n，较长的那个先走N步，然后和较短的那个同时遍历。如果指针指向同一元素，则为交点，否则A和B 没有交点。该方法能够满足线性时间复杂度和常数空间复杂度的要求。

/**
 * 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) 
    {
         int lenA=0,lenB=0;
         ListNode *p=headA;
         while(p)
         {
             p=p->next;
             lenA++;
         }
         p=headB;
         while(p)
         {
             p=p->next;
             lenB++;
         }
         
         if(lenA>=lenB)
         {
             p=headA;
             for(int i=0;i<lenA-lenB;i++)
                p=p->next;
             ListNode *q=headB;    
             while(p && q)
             {
                 if(p==q)
                   return p;
                 p=p->next;
                 q=q->next;
             }
             return NULL;
         }
         else
         {
             p=headB;
             for(int i=0;i<lenB-lenA;i++)
                p=p->next;
             ListNode *q=headA;    
             while(p && q)
             {
                 if(p==q)
                   return p;
                 p=p->next;
                 q=q->next;
             }
             return NULL;
         }
    }
};