合併兩個排序後的鏈表

算法描述：
輸入兩個單調遞增的鏈表，輸出兩個鏈表合成後的鏈表，當然我們需要合成後的鏈表滿足單調不減規則。

思路：
1. 兩條鏈表list1+list21分別單調遞增；
2. 選擇較小的鏈表頭作爲合併後的頭結點；
3. 再兩兩進行比較，知道兩節點大小關係反轉；
4. 時間複雜度O(n),空間複雜度O(1);
5. helper1程序看起來簡單，但其中的每個節點要比較兩次，不建議使用，建議helper2；
6. helper3通過牛客網上的test case，但是不能通過這裏的case，wrong。放這裏供參考；
7. 這裏的程序copy下來完全可以跑；

public class Main {
    public static void main(String[] args) {
        ListNode list1 = new ListNode(1), list2 = new ListNode(2), list3 = new ListNode(3), list4 = new ListNode(7),
                list5 = new ListNode(9);
        list1.next = list2;
        list2.next = list3;
        list3.next = list4;
        list4.next = list5;

        ListNode head = list1;
        while (head != null) {
            System.out.print(head.val + " ");
            head = head.next;
        }
        System.out.println(" ");

        ListNode list21 = new ListNode(0), list22 = new ListNode(4), list23 = new ListNode(5), list24 = new ListNode(6),
                list25 = new ListNode(10);
        list21.next = list22;
        list22.next = list23;
        list23.next = list24;
        list24.next = list25;

        head = list21;
        while (head != null) {
            System.out.print(head.val + " ");
            head = head.next;
        }
        System.out.println(" ");

        head = Merge(list1, list21);
        while (head != null) {
            System.out.print(head.val + " ");
            head = head.next;
        }
        System.out.println(" ");

    }
    public static ListNode Merge(ListNode list1, ListNode list2) {
        if (list1 == null)
            return list2;
        if (list2 == null)
            return list1;
        if (list1.val < list2.val)
            return helper1(list1, list2);
        return helper1(list2, list1);
    }

    public static ListNode helper1(ListNode head1, ListNode head2) {
        ListNode head = head1, next = null;
        while (head1 != null && head2 != null) {
            if (head1.val <= head2.val) {
                if (head1.next != null && head1.next.val <= head2.val) {
                    head1 = head1.next;
                    continue;
                }
                next = head1.next;
                head1.next = head2;
                head1 = next;
            } else {
                if (head2.next != null && head2.next.val < head1.val) {
                    head2 = head2.next;
                    continue;
                }
                next = head2.next;
                head2.next = head1;
                head2 = next;
            }
        }

        return head;
    }

    public static ListNode helper2(ListNode head1, ListNode head2) {
        ListNode head = head1, next = null;
        while (head1 != null && head2 != null) {
            if (head1.val <= head2.val) {
                while (head1.next != null && head2 != null) {
                    if (head1.next.val <= head2.val) {
                        head1 = head1.next;
                    } else
                        break;
                }
                next = head1.next;
                head1.next = head2;
                head1 = next;
            } else {
                while (head1 != null && head2.next != null) {
                    if (head1.val > head2.next.val) {
                        head2 = head2.next;
                    } else
                        break;
                }
                next = head2.next;
                head2.next = head1;
                head2 = next;
            }
        }

        return head;
    }

    public static ListNode helper3(ListNode head1, ListNode head2) {
        ListNode head = head1, next1 = null, next2 = null;
        while (head1 != null && head2 != null) {
            if (head1.val <= head2.val) {
                next1 = head1.next;
                head1.next = head2;
                head1 = next1;
            } else {
                next2 = head2.next;
                head2.next = head1;
                head2 = next2;
            }
        }
        return head;
    }
   }

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