合并两个有序链表

/**
 * @author liizzz
 * @date 2019-10-24
 */
public class S17 {

    public static void main(String[] args) {
        S17 s17 = new S17();
        ListNode list1 = s17.init1();
        ListNode list2 = s17.init2();

        ListNode retList = s17.merge(list1, list2);
        ListNode temp = retList;
        System.out.println("合并后的链表为： ");
        while (temp != null) {
            System.out.print(temp.val + " --> ");
            temp = temp.next;
        }
        System.out.println();
    }

    public ListNode init1() {
        ListNode head = new ListNode(0);
        ListNode node = head;
        for (int i = 2; i < 10; i = i + 2) {
            node.next = new ListNode(i);
            node = node.next;
        }

        System.out.println("list1 链表为：");
        ListNode temp = head;
        while (temp != null) {
            System.out.print(temp.val + " --> ");
            temp = temp.next;
        }
        System.out.println();
        return head;
    }

    public ListNode init2() {
        ListNode head = new ListNode(1);
        ListNode node = head;
        for (int i = 3; i <= 10; i = i + 2) {
            node.next = new ListNode(i);
            node = node.next;
        }
        System.out.println("list2 链表为：");
        ListNode temp = head;
        while (temp != null) {
            System.out.print(temp.val + " --> ");
            temp = temp.next;
        }
        System.out.println();
        return head;
    }

    public ListNode merge(ListNode list1, ListNode list2) {
        if (list1 == null) {
            return list2;
        }
        if (list2 == null) {
            return list1;
        }

        ListNode root;
        if (list1.val < list2.val) {
            root = new ListNode(list1.val);
            list1 = list1.next;
        } else {
            root = new ListNode(list2.val);
            list2 = list2.next;
        }
        ListNode node = root;
        while (list1 != null && list2 != null) {
            if (list1.val < list2.val) {
                //list1 前进1
                node.next = new ListNode(list1.val);
                node = node.next;
                list1 = list1.next;
            } else {
                node.next = new ListNode(list2.val);
                node = node.next;
                list2 = list2.next;
            }
        }

        while (list1 != null) {
            node.next = new ListNode(list1.val);
            node = node.next;
            list1 = list1.next;
        }
        while (list2 != null) {
            node.next = new ListNode(list2.val);
            node = node.next;
            list2 = list2.next;
        }

        return root;
    }

    class ListNode {
        int val;
        ListNode next;

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

        public ListNode() {
        }
    }
}