public class TwoLinkPublicNode52 {
    static class Node {
        int val;
        Node next;
        public Node(int val){
            this.val = val;
        }
        @Override
        public String toString() {
            return "Node{" +
                    "val=" + val +
                    ", next=" + next +
                    '}';
        }
    }
    public static void main(String[] args) {
        Node node11 = new Node(1);
        Node node12 = new Node(2);
        Node node13 = new Node(3);
        Node node14 = new Node(4);
        node11.next = node12;
        node12.next = node13;
        node13.next = node14;

        Node node20 = new Node(0);
        Node node21 = new Node(2);
        Node node22 = new Node(1);
        Node node23 = new Node(3);
        Node node24 = new Node(4);
        node20.next = node21;
        node21.next = node22;
        node22.next = node23;
        node23.next = node24;

        System.out.println(findFirst(node11, node20).val);

    }

    private static Node findFirst(Node root1, Node root2) {
        Node p1 = root1, p2 = root2;
        int num1 = 0, num2 = 0;
        while (p1 != null) {
            num1++;
            p1 = p1.next;
        }
        while (p2 != null) {
            num2++;
            p2 = p2.next;
        }
        p1 = root1;
        p2 = root2;
        int diff = 0;
        if (num2 > num1) {
            diff = num2 - num1;
            while (diff != 0) {
                p2 = p2.next;
                diff--;
            }
        } else {
            diff = num1 - num2;
            while (diff != 0) {
                p1 = p1.next;
                diff--;
            }
        }

        while (p1.val != p2.val) {
            p1 = p1.next;
            p2 = p2.next;
        }
        return p1;
    }
}

解释：易知第一个公共节点往后的所有节点两个链表都一样，长度也一样；那么计算出两个链表长度的差minus，较长的链表先往前走minus步，然后两个链表同时往前走，边走边比较，直到找到第一个相等的节点