Merge2SortedList.java

package learn.freq05;

import entity.ListNode;

//Merge two sorted linked lists and return it as a new list.
//The new list should be made by splicing together the nodes of the first two lists.
//把2个sort好的linkedlist合并成一个sort好的linkedlist  类似 mergesort
public class Merge2SortedList {

    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if (l1 == null && l2 == null) {
            return null;
        }
        if (l1 == null && l2 != null) {
            return l2;
        }
        if (l1 != null && l2 == null) {
            return l1;
        }
        // 这个root 是dummy head 为了方便编程，是空的，最后 返回的是head.next
        ListNode head = new ListNode(0); // root 到最后会变成指向linkedlist倒数第二个节点
        ListNode root = head;
        while (l1 != null && l2 != null) {
            //不要忘了 都是先建好 root.next = 你要的东西 然后再把 root.next设为root 下次实际上等于是说root。next。next=。。。。
            if (l1.val <= l2.val) {
                root.next = new ListNode(l1.val);
                root = root.next;
                l1 = l1.next;
            } else {
                root.next = new ListNode(l2.val);
                root = root.next;
                l2 = l2.next;
            }

        }
        if (l1 == null) {
            if (l2 != null) {
                root.next = l2;
            }
        } else if (l2 == null) {
            if (l1 != null) {
                root.next = l1;
            }
        }

        return head.next;
    }

    public static void main(String[] args) {
        ListNode root1 = new ListNode(2);

        ListNode root2 = new ListNode(1);
        System.out.print((new Merge2SortedList().mergeTwoLists(root1, root2)).val);
    }

}