每天一道剑指offer-两个链表的第一个公共结点

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode FindFirstCommonNode(ListNode pHead1, ListNode pHead2) {
        if(pHead1 == null || pHead2 == null)
            return null;
        int length1 = 0;
        int length2 = 0;
        ListNode p1 = pHead1,p2 = pHead2;
        while(p1 != null)
        {
            length1++;
            p1 = p1.next;
        }
        while(p2 != null)
        {
            length2++;
            p2 = p2.next;
        }//分别求两个链表长度
        int cha = 0;
        if(length1 > length2)
        {//求长度差，长的先走长度差个单位
            cha = length1 - length2;
            p1 = pHead1;
            p2 = pHead2;
            for(int i=0;i<cha;i++)
                p1 = p1.next;
        }else{
            cha = length2 - length1;
            p2 = pHead2;
            p1 = pHead1;
            for(int i=0;i<cha;i++)
                p2 = p2.next;
        }
        while(p1 != p2)
        {//找第一个相同的节点
            p1 = p1.next;
            p2 = p2.next;
        }
        return p1;
    }
}