[LeetCode] 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.

 

/* Definition for singly-linked list.
 * 
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
    int getLen(ListNode *p)
    {
        int len = 0;
        while(p)
        {
            p=p->next;
            len++;
        }
        return len;
    }
public:
    ListNode *getIntersectionNode(ListNode *headA, 
                                  ListNode *headB) 
    {
        int lenA = getLen(headA);
        int lenB = getLen(headB);
        while(lenA>lenB)
        {
            lenA--;
            headA = headA->next;
        }
        while(lenA<lenB)
        {
            lenB--;
            headB = headB->next;
        }
        while(headA != headB)
        {
            headA = headA->next;
            headB = headB->next;
        }
        return headA;
    }
};