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. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { *         val = x; *         next = null; *     } * } */public class Solution {    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {        if(headA == null ||headB == null)return null;          ListNode h1 = headA;          ListNode h2 = headB;          int count1 = 1, count2 = 1;          while(h1.next != null){              count1++;              h1 = h1.next;          }          while(h2.next != null){              count2++;              h2 = h2.next;          }          if(h1 != h2) return null;          else{              int count = Math.abs(count1 - count2);              if(count2 > count1){                  h1 = headB;                  h2 = headA;               }              else{                  h1 = headA;                  h2 = headB;              }              while((count--) > 0){                  h1 = h1.next;              }              while(h1 != null&&h2 != null && h1 != h2){                  h1 = h1.next;                  h2 = h2.next;              }              return h1;          }      }}