[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 {
  public:
      ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
          ListNode *p1 = headA;
          ListNode *p2 = headB;
          int cnt1 = 0,cnt2 = 0;
          while(p1){
              cnt1 ++;
              p1 = p1 -> next;
          }
          while(p2){
              cnt2 ++;
              p2 = p2 -> next;
          }
          p1 = cnt1 >= cnt2 ? headA : headB;
          p2 = cnt1 >= cnt2 ? headB : headA;
          int cnt = abs(cnt1 - cnt2);
          while(cnt --)
              p1 = p1 -> next;
          while(p1){
              if(p1 -> val == p2 -> val)
                  return p1;
              p1 = p1 -> next;
              p2 = p2 -> next;
          }
          return NULL;
      }
  };