剑指offer:面试题17、合并两个排序的链表

题目描述

输入两个单调递增的链表，输出两个链表合成后的链表，当然我们需要合成后的链表满足单调不减规则。

代码示例

public class Offer17 {
    public static void main(String[] args) {
        ListNode l1 = new ListNode(1);
        l1.next = new ListNode(2);
        l1.next.next = new ListNode(2);
        ListNode l2 = new ListNode(3);
        l2.next = new ListNode(4);
        Offer17 testObj = new Offer17();
        testObj.printList(l1);
        testObj.printList(l2);
//        testObj.printList(testObj.mergeTwoList(l1,l2));
        testObj.printList(testObj.mergeTwoList2(l1,l2));

    }

    //递归合并两个链表
    public ListNode mergeTwoList(ListNode l1, ListNode l2) {
        if (l1 == null) {
            return l2;
        }
        if (l2 == null) {
            return l1;
        }

        if (l1.val <= l2.val) {
            l1.next = mergeTwoList(l1.next, l2);
            return l1;
        } else {
            l2.next = mergeTwoList(l1, l2.next);
            return l2;
        }
    }

    //迭代合并两个链表
    public ListNode mergeTwoList2(ListNode l1, ListNode l2) {
        ListNode head = new ListNode(-1);
        ListNode cur = head;
        while (l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                cur.next = l1;
                l1 = l1.next;
            } else {
                cur.next = l2;
                l2 = l2.next;
            }
            cur = cur.next;
        }
        if (l1 != null)
            cur.next = l1;
        if (l2 != null)
            cur.next = l2;
        return head.next;
    }
    //打印链表
    public void printList(ListNode head) {
        if (head == null) {
            return;
        }
        while (head != null) {
            System.out.print(head.val + " ");
            head = head.next;
        }
        System.out.println();
    }

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