LeetCode:Intersection of Two Linked Lists

Intersection of Two Linked Lists

Total Accepted: 74850  Total Submissions: 246823  Difficulty: Easy

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.

Credits:
Special thanks to @stellari for adding this problem and creating all test cases.

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c++ code:

/**
 * 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 || !headB) return NULL;
        int cntA=0,cntB=0;
        ListNode *pa=headA, *pb=headB;
        while(pa)
        {
            pa=pa->next;
            cntA++;
        }
        while(pb)
        {
            pb=pb->next;
            cntB++;
        }
        pa=headA,pb=headB;
        while(cntB < cntA) 
        {
            pa=pa->next;
            cntA--;
        }
        while(cntA < cntB) 
        {
            pb=pb->next;
            cntB--;
        }
        while(pa && pb) {
            if(pa == pb) break;
            else{
                pa=pa->next;
                pb=pb->next;
            }
        }
        return pa;
    }
};