剑指offer：两个链表的第一个公共节点（java）

/**
 * 题目：
 *      输入两个链表，找出它们的第一个公共结点。
 * 解题思路：
 *      首先求出两个链表的长度，获得两个链表的长度差differ,
 *      在第二次遍历时，长链表先走differ步，然后同时遍历两个链表，
 *      找到第一个相同的节点就是他们的第一个公共节点。
 */
public class P253_FirstCommonNodeBetweenTwoLinkedlists {
    public ListNode FirstCommonNodeBwtweenLinkedlists(ListNode pHead1, ListNode pHead2) {
        ListNode result = null;

        if (pHead1 == null || pHead2 == null) {
            return result;
        }

        //分别求出两个链表的长度
        int length1 = GetLength(pHead1);
        int length2 = GetLength(pHead2);

        ListNode longList = null;
        ListNode shortList = null;

        //找出pHead1和pHead2哪个是长链表、短链表
        if (length1 > length2) {
            longList = pHead1;
            shortList = pHead2;
        }
        else {
            longList = pHead2;
            shortList = pHead1;
        }

        //求出两个链表的长度差，并让长链表先走differ步
        int differ = Math.abs(length1 - length2);
        for (int i = 0; i < differ; i++) {
            longList = longList.next;
        }

        //同时遍历两个链表，寻找第一个相同的节点
        while (longList != null && shortList != null) {
            if (longList == shortList) {
                result = longList;
                break;
            }
            else {
                longList = longList.next;
                shortList = shortList.next;
            }
        }
        return result;

    }

    public int GetLength(ListNode pHead) {
        if (pHead == null) {
            return 0;
        }
        int count = 0;
        ListNode node = pHead;
        while (node != null) {
            count++;
            node = node.next;
        }
        return count;
    }

    public static void main(String[] args) {
        ListNode node1 = new ListNode(1);
        ListNode node2 = new ListNode(2);
        ListNode node3 = new ListNode(3);
        ListNode node4 = new ListNode(4);
        ListNode node5 = new ListNode(5);
        ListNode node6 = new ListNode(6);
        ListNode node7 = new ListNode(7);

        node1.next = node2;
        node2.next = node3;
        node3.next = node6;
        node6.next = node7;

        node4.next = node5;
        node5.next = node6;


        P253_FirstCommonNodeBetweenTwoLinkedlists test = new P253_FirstCommonNodeBetweenTwoLinkedlists();
        ListNode result = test.FirstCommonNodeBwtweenLinkedlists(node1, node4);
        System.out.println(result.val);
    }
}