剑指Offer面试题17 & Leetcode21

剑指Offer面试题17 & Leetcode21

Merge Two Sorted Lists  合并两个排序的链表

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.

解题思路

  考虑：链表1的头结点和链表2的头结点中的较小者为合并后链表的头结点，之后合并的是链表1第二个节点开始的链表和链表2，可以用递归的思想解决问题。注意处理空指针问题。当然也可以用循环，用一个ListNode保留头节点，然后用另一个ListNode遍历两个链表。

Solution1 递归

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {  
        if(l1 == null)
            return l2;
        if(l2 == null)
            return l1;
        ListNode head;
        if(l1.valelse{
            head = l2;
            head.next = mergeTwoLists(l1, l2.next);
        }
        return head;
}

Solution2 循环

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {  
        if (l1 == null) {  
             return l2;  
         }  
         if (l2 == null) {  
             return l1;  
         }  
         ListNode node = null;  
         ListNode head = null;  
         while (l1 != null && l2 != null) {  
             if (l1.val <= l2.val) {  
                 if (node == null) {  
                     node = l1;  
                     head = node;  
                 } else {  
                     node.next = l1;  
                     node = node.next;  
                 }  
                 l1 = l1.next;  
             } else {  
                 if (node == null) {  
                     node = l2;  
                     head = node;  
                 } else {  
                     node.next = l2;  
                     node = node.next;  
                 }  
                 l2 = l2.next;  
        }  
         }  
         if (l1 != null) {  
             node.next = l1;  
         } else if (l2 != null) {  
             node.next = l2;  
         }  
         return head;  
    }