题解 | #两个链表的第一个公共结点#

import java.util.*;
/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode FindFirstCommonNodeV1(ListNode pHead1, ListNode pHead2) {
        Deque<ListNode> queue = new LinkedList<>();

        while (pHead1  != null) {
            queue.add(pHead1);
            pHead1 = pHead1.next;
        }

        while (pHead2 != null) {
            if (queue.contains(pHead2)) {
                return pHead2;
            }
            pHead2 = pHead2.next;
        }

        return null;
    }

    public ListNode FindFirstCommonNode(ListNode pHead1, ListNode pHead2) {
        int lengthA = 0;
        int lengthB = 0;
        ListNode pHeadA = pHead1;
        ListNode pHeadB = pHead2;
        while (pHeadA != null) {
            pHeadA = pHeadA.next;
            lengthA++;
        }

        while (pHeadB != null) {
            pHeadB = pHeadB.next;
            lengthB++;
        }

        pHeadA = pHead1;
        pHeadB = pHead2;

        //长的链表先走
        if (lengthA > lengthB) {
            for (int i = 0; i < lengthA - lengthB; i++) {
                pHeadA = pHeadA.next;
            }
        } else {
            for (int i = 0; i < lengthB - lengthA; i++) {
                pHeadB = pHeadB.next;
            }
        }

        //同时向前移动指针，直到相等。若到尾节点，则没有相同的。
        while (pHeadA != pHeadB) {
            pHeadA = pHeadA.next;
            pHeadB = pHeadB.next;
        }

        return pHeadA;
    }
}