基础算法面试题---合并有序链表

将两个升序链表合并为一个新的 升序 链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。

原链表：l1 = [1,2,4]，l2 = [1,3,4]
合并后：[1,1,2,3,4,4]

public class Code_04 {
    
    

    public static void main(String[] args) {
    
    
        ListNode n = new ListNode(1);
        ListNode l1 = n;
        for (int i = 3; i <= 5; i = i + 2) {
    
    
            n.next = new ListNode(i);
            n = n.next;
        }
        ListNode n2 = new ListNode(2);
        ListNode l2 = n2;
        for (int i = 4; i <= 6; i = i + 2) {
    
    
            n2.next = new ListNode(i);
            n2 = n2.next;
        }
        Code_04 c = new Code_04();

        System.out.println("原链表1：" + l1);
        System.out.println("原链表2：" + l2);

        System.out.println("合并后：" + c.mergeTwoLists(l1, l2));

    }

    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
    
    
        if (l1 == null) {
    
    
            return l2;
        }
        if (l2 == null) {
    
    
            return l1;
        }
        ListNode mergeNode = new ListNode();
        ListNode pre = mergeNode;
        //比较两个链表当前的值，值小的链表就把引用赋给mergeNode，并向后移动一位重新赋值给自己，同时pre指向值小的那个节点
        while (l1 != null && l2 != null) {
    
    
            if (l1.val <= l2.val) {
    
    
                pre.next = l1;
                l1 = l1.next;
            } else {
    
    
                pre.next = l2;
                l2 = l2.next;
            }
            pre = pre.next;
        }
        pre.next = l1 == null ? l2 : l1;

        return mergeNode.next;
    }
}