首先计算出两个链表的长度，求长度之差，先让长度长的走差值个步数，然后前后两个指针一块向前走，到两个指针指向的值相同。

public class Solution {
    public int lengthOfListnode(ListNode head){
        if (head==null) return 0;
        int length = 0;
        while(head!=null){
            length++;
            head=head.next;
        }
        return length;
    }
    public ListNode FindFirstCommonNode(ListNode pHead1, ListNode pHead2) {
        int length_1 = lengthOfListnode(pHead1);
        int length_2 = lengthOfListnode(pHead2);
        if(length_2>length_1){
            ListNode temp = pHead1;
            pHead1=pHead2;
            pHead2=temp;
        }
        int minus = Math.abs(length_1-length_2);
        ListNode fast=pHead1;
        ListNode slow=pHead2;
        for (int i=0;i<minus;i++){
            fast = fast.next;
        }
        while(fast!=null&&slow!=null){
            if(fast.val==slow.val){
                return fast;
            }
            fast = fast.next;
            slow = slow.next;
        }
        return null;
    }
}