输入两个单向链表，找出它们的第一个公共结点

题目：

输入两个单向链表，找出它们的第一个公共结点

解答：

public class Solution {


    public static void main(String[] args) {
        ListNode head1 = new ListNode();
        ListNode second1 = new ListNode();
        ListNode third1 = new ListNode();
        ListNode forth1 = new ListNode();
        ListNode fifth1 = new ListNode();
        ListNode head2 = new ListNode();
        ListNode second2 = new ListNode();
        ListNode third2 = new ListNode();
        ListNode forth2 = new ListNode();
        head1.nextNode = second1;
        second1.nextNode = third1;
        third1.nextNode = forth1;
        forth1.nextNode = fifth1;
        head2.nextNode = second2;
        second2.nextNode = forth1;
        third2.nextNode = fifth1;
        head1.data = 1;
        second1.data = 2;
        third1.data = 3;
        forth1.data = 6;
        fifth1.data = 7;
        head2.data = 4;
        second2.data = 5;
        third2.data = 6;
        forth2.data = 7;
        System.out.println(findFirstCommonNode(head1,head2).data);
        
    }

    public static ListNode findFirstCommonNode(ListNode head1, ListNode head2) {
        int len1 = getListLength(head1);
        int len2 = getListLength(head2);

        ListNode longListNode = null;
        ListNode shortListNode = null;

        int diff = 0;
        if(len1 > len2) {
            longListNode = head1;
            shortListNode = head2;
            diff = len1 - len2;
        } else {
            longListNode = head2;
            shortListNode = head1;
            diff = len2 - len1;
        }

        for(int i = 0; i < diff; i++) {
            longListNode = longListNode.nextNode;
        }

        while(longListNode != null && shortListNode != null && longListNode != shortListNode) {
            longListNode = longListNode.nextNode;
            shortListNode = shortListNode.nextNode;
        }

        return longListNode;
    }



    private static int getListLength(ListNode head) {
        int result = 0;
        if(head == null) {
            return result;
        }

        ListNode point = head;
        while(point != null) {
            point = point.nextNode;
            result++;
        }

        return result;
    }
}


public class ListNode {
    int data;
    ListNode nextnode;
}