【LeetCode】160

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.

Tags : Linked List

Solution : notice: it's intersection, not equal of value

/**
 * 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) {
        //pre
        if(headA==NULL||headB==NULL)
            return NULL;
        //length
        ListNode *a=headA,*b=headB;
        int lenA=0,lenB=0;
        while(a!=NULL){
            lenA++;
            a=a->next;
        }
        while(b!=NULL){
            lenB++;
            b=b->next;
        }
        if(lenA>lenB){
            for(int i=0;i<lenA-lenB;i++){
                headA=headA->next;
            }
        }else{
            for(int i=0;i<lenB-lenA;i++){
                headB=headB->next;
            }
        }
        while(headA!=headB){
            headA=headA->next;
            headB=headB->next;
        }
        return headA;
    }
};