代码如下：

package demo;

/**
 * 两个链表的第1个公共结点
 * 
 * @author xiangdonglee
 *
 */
public class Test37 {
    private static class ListNode {
        int val;
        ListNode next;

        public ListNode() {
        }

        public ListNode(int val) {
            this.val = val;
        }

        @Override
        public String toString() {
            return val + "";
        }
    }

    public static ListNode findFirstCommonNode(ListNode head1, ListNode head2) {
        // 得到2个链表的长度
        int length1 = getListLength(head1);
        int length2 = getListLength(head2);
        int diff = length1 - length2;
        ListNode longListHead = head1;
        ListNode shortListHead = head2;
        if (diff < 0) {
            longListHead = head2;
            shortListHead = head1;
            diff = length2 - length1;
        }
        // 先在长链表上走几步，再同时在2个链表上遍历
        for (int i = 0; i < diff; i++) {
            longListHead = longListHead.next;
        }
        while (longListHead != null && shortListHead != null && longListHead != shortListHead) {
            longListHead = longListHead.next;
            shortListHead = shortListHead.next;
        }
        // 返回第一个公共结点，如果没有就返回null
        return longListHead;
    }

    private static int getListLength(ListNode head) {
        int result = 0;
        while (head != null) {
            result++;
            head = head.next;
        }
        return result;
    }

    public static void main(String[] args) {
        test1();
        test2();
        test3();
        test4();
        test5();
    }

    /**
     * 第1个公共结点在链表中间
     */
    private static void test1() {
        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;
        System.out.println(findFirstCommonNode(node1, node4));
    }

    /**
     * 没有公共结点
     */
    private static void test2() {
        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 = node4;
        node5.next = node6;
        node6.next = node7;
        System.out.println(findFirstCommonNode(node1, node5));
    }

    /**
     * 公共结点是最后一个结点
     */
    private static void test3() {
        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 = node4;
        node4.next = node7;
        node5.next = node6;
        node6.next = node7;
        System.out.println(findFirstCommonNode(node1, node5));
    }

    /**
     * 公共结点是第一个结点，2个链表完全重合
     */
    private static void test4() {
        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 = node4;
        node4.next = node5;
        node5.next = node6;
        node6.next = node7;
        System.out.println(findFirstCommonNode(node1, node1));
    }

    /**
     * 输入的链表头结点是null指针
     */
    private static void test5() {
        System.out.println(findFirstCommonNode(null, null));
    }
}