合并两个有序链表

• 非递归实现

/**
 * Definition for singly-linked list.
 * struct ListNode 
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
 
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode* header = new ListNode(-1);
        ListNode* cur = header;
        while(l1 || l2)
        {
            int n = 0;
            if(l1 == NULL || (l1 && l2 && l1->val > l2->val))
            {
                n = l2->val;
                l2 = l2->next;
            }
            else if(l2 == NULL || (l1 && l2 && l1->val <= l2->val))
            {
                n = l1->val;
                l1 = l1->next;
            }

            cur->next = new ListNode(n);
            cur = cur->next;
        }

        ListNode *ans = header->next;
        delete header;
        return ans;
    }
};

• 递归实现

Node* MergeListR(Node* Head1,Node* Head2)
{
    if(NULL == Head1 || NULL == Head2){
        if(NULL == Head1)
            return Head2;

        return Head1;
    }

    Node* newHead = NULL;
    //较小的作为新链表的头
    if(Head1->_data < Head2->_data){
        newHead = Head1;
        newHead->_next = MergeList(newHead->_next,Head2);
    }else{
        newHead = Head2;
        newHead->_next = MergeList(Head1,newHead->_next);
    }

    return newHead;
}