题解 | #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 len1 = 0, len2 = 0;
	    //这里需要定义两个临时的头节点，不能直接使用pHead1和pHead2。
        ListNode tmp1 = pHead1, tmp2 = pHead2;
        while(tmp1 != null){
            tmp1 = tmp1.next;
            len1++;
        }
        while(tmp2 != null){
            tmp2 = tmp2.next;
            len2++;
        }

        if(len1 > len2){
            //pHead1要长一些。
            for(int i = 0; i < len1 - len2; i++){
                pHead1 = pHead1.next;
            }
        }else{
            //pHead2要长一些
            for(int i = 0; i < len2 - len1; i++){
                pHead2 = pHead2.next;
            }
        }

        while(pHead1 != null && pHead2 != null){
            if(pHead1 == pHead2){
                return pHead1;
            }

            pHead1 = pHead1.next;
            pHead2 = pHead2.next;
        }

        return null;
 
    }
}