两个链表的第一个公共结点

题目：输入两个链表，找出它们的第一个公共结点。

（1）遍历两个链表，记录两个链表的长度。先遍历长链表，遍历到与短链表长度相同的位置，再开始比较两个链表的结点是否相同。

public class Solution {
    public ListNode FindFirstCommonNode(ListNode pHead1, ListNode pHead2) {
        int len1 = 1, len2 = 1;
        if(pHead1 == null || pHead2 == null)
            return null;
        ListNode L1 = pHead1, L2 = pHead2;
        while(L1.next != null){
            len1++;
            L1 = L1.next;
        }
        while(L2.next != null){
            len2++;
            L2 = L2.next;
        }
        if(len1 > len2){
            int count = len1 - len2;
            while(count-- > 0){
                pHead1 = pHead1.next;
            }
        }else{
            int count = len2 - len1;
            while(count-- >0)
                pHead2 = pHead2.next;
        }
        while(pHead1 != null){
            if(pHead1 == pHead2)
                return pHead1;
            else{
                pHead1 = pHead1.next;
                pHead2 = pHead2.next;
            }
        }
        return null;
    }
}

（2）利用hashMap的特性

public class Solution {
    public ListNode FindFirstCommonNode(ListNode pHead1, ListNode pHead2) {
        ListNode current1 = pHead1;
        ListNode current2 = pHead2;
 
 
        HashMap<ListNode, Integer> hashMap = new HashMap<ListNode, Integer>();
        while (current1 != null) {
            hashMap.put(current1, null);
            current1 = current1.next;
        }
        while (current2 != null) {
            if (hashMap.containsKey(current2))
                return current2;
            current2 = current2.next;
        }
 
        return null;
 
    }
}