两个链表的第一个公共结点

• 输入两个链表，找出它们的第一个公共结点。（注意因为传入数据是链表，所以错误测试数据的提示是用其他方式显示的，保证传入数据是正确的）
• 计算出两个差表的长度，同时开始查询
• C++ 代码

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

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode FindFirstCommonNode(ListNode pHead1, ListNode pHead2) {
        int len1 = countLength(pHead1);
        int len2 = countLength(pHead2);
        if(len1 > len2) {
            int diff = len1 - len2;
            while((diff--)>0){
                pHead1 = pHead1.next; 
            }
        } else if(len2 > len1){
            int diff = len2 - len2;
            while((diff--)>0){
                pHead2 = pHead2.next; 
            }
        }
        return commomHead(pHead1, pHead2);     
    }
    
    private int countLength(ListNode head) {
        int len = 0;
        ListNode p = head;
        while(p != null) {
            len++;
            p = p.next;
        }
        return len;
    }
    
    private ListNode commomHead(ListNode pHead1, ListNode pHead2){
        while(pHead1!=null && pHead2!=null) {
            if(pHead1 == pHead2) return pHead1;
            else {
                pHead1 = pHead1.next;
                pHead2 = pHead2.next;
            }
        }
        return null;
    }
}