[LeetCode] 21. Merge Two Sorted Lists 解题思路

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.

问题：将两个已排序的列表，合并为一个有序列表。

令 head 为两个列表表头中较小的一个，令 p 为新的已排序的最后一个元素。令 l1, l2 分别为两个列表中未排序部分的首节点。依次将 l1, l2 中的较小值追加到 p 后面，并调整 p 和 l1、l2较小者指针即可。

    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        
        if (l1 == NULL) {
            return l2;
        }
        
        if (l2 == NULL) {
            return l1;
        }
        
        ListNode* head = new ListNode(0);
        if (l1->val <= l2->val) {
            head = l1;
            l1 = l1->next;
        }else{
            head = l2;
            l2 = l2->next;
        }
        
        ListNode* p = head;
        
        while(l1 != NULL && l2 != NULL){
            
            if(l1->val <= l2->val){
                p->next = l1;
                p = p->next;
                l1 = l1->next;
            }else{
                p->next = l2;
                p = p->next;
                l2 = l2->next;
            }
        }
                
        if (l1 == NULL) {
            p->next = l2;
        }
        
        if (l2 == NULL) {
            p->next = l1;
        }
        
        return head;
    }