AcWing 36. 合并两个排序的链表（归并+混洗）

1.归并

具体代码如下：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* merge(ListNode* l1, ListNode* l2) {
        ListNode *head = new ListNode(-1); //临时头结点或伪头结点
        ListNode *p = head;
        while(l1 && l2){
            if(l1 -> val < l2 -> val){
                p -> next = l1;
                l1 = l1 -> next;
            }
            else{
                p -> next = l2;
                l2 = l2 -> next;
            }
            p = p -> next;
        }
        if(l1) p -> next = l1;
        if(l2) p -> next = l2;
        return head -> next;
    }  
};

2.混洗后排序再连接

将两个链表中所有结点存入同一个优先级队列中（或者向量中然后排序），然后出队的同时将各个结点相连即可。

具体代码如下：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    struct cmp{
        bool operator () (const ListNode *a, const ListNode *b){
            return a -> val > b -> val;
        }
    };

    ListNode* merge(ListNode* l1, ListNode* l2) {
        if(!l1 && !l2) return NULL;
        priority_queue<ListNode *, vector<ListNode *>, cmp> q;
        while(l1){
            q.push(l1);
            l1 = l1 -> next;
        }
        while(l2){
            q.push(l2);
            l2 = l2 -> next;
        }
        ListNode *head = q.top(), *p;
        p = head;
        q.pop();
        while(!q.empty()){
            p -> next = q.top();
            q.pop();
            p = p -> next;
        }
        p -> next = NULL;
        return head;
    }  
};