leetcode 160. Intersection of Two Linked Lists

原文鏈接：http://blog.csdn.net/litoupu/article/details/41555557

題目

Write a program to find the node at which the intersection of two singly linked lists begins.

For example, the following two linked lists:

begin to intersect at node c1.

Notes:

1，If the two linked lists have no intersection at all, return null.
2，The linked lists must retain their original structure after the function returns.
3，You may assume there are no cycles anywhere in the entire linked structure.
4，Your code should preferably run in O(n) time and use only O(1) memory.

解

查找兩個鏈表的第一個公共節點，如果兩個節點的尾節點相同，肯定存在公共節點
方法： 長的鏈表開始多走 （h1的數量 - h2的數量）步，然後和短鏈表同步往下走，遇到的第一個相同的節點就是最早的公共節點

/**
 * 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;
       int 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(count1>count2){
               h1=headA;
               h2=headB;
           }else{
               h1=headB;
               h2=headA;
           }
           while(count>0){
               h1=h1.next;
               count--;
           }

           while(h1!=null && h2!=null && h1!=h2){
               h1=h1.next;
               h2=h2.next;
           }

           return h1;
       }

      }
}