剑指offer---合并两个排序的链表

问题：合并两个排序的链表

要求：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。

struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
            val(x), next(NULL) {
    }
};

class Solution {
public:
    ListNode* Merge(ListNode* pHead1, ListNode* pHead2){

    }
};

 思路：

 

采用两个指针指向两链表，比较指针节点的大小，值较小的节点肯定需要放到靠前的位置，同时指针指向下一个节点；

解题代码：

递归版：

struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
            val(x), next(NULL) {
    }
};

class Solution {
public:
    ListNode* Merge(ListNode* pHead1, ListNode* pHead2){
        if(pHead1 == nullptr)
            return pHead2;
        else if(pHead2 == nullptr)
            return pHead1;
        // 两个链表都不为空
        ListNode *pMerge = nullptr;
        if(pHead1->val < pHead2->val){
            pMerge = pHead1;
            pMerge->next = Merge(pHead1->next, pHead2);
        }
        else{
            pMerge = pHead2;
            pMerge->next = Merge(pHead1, pHead2->next);
        }
        return pMerge;
    }
};

 非递归版：

class Solution {
public:
    ListNode* Merge(ListNode* pHead1, ListNode* pHead2){
        if(pHead1 == nullptr) return pHead2;
        if(pHead2 == nullptr) return pHead1;

        ListNode* pHead = nullptr;
        if(pHead1->val < pHead2->val){
            pHead = pHead1;
            pHead1 = pHead1->next;
        }
        else{
            pHead = pHead2;
            pHead2 = pHead2->next;
        }
        ListNode*temp = pHead;
        // 排序合并，直到较短的链表节点全部遍历完毕
        while(pHead1 != nullptr && pHead2 != nullptr){
            if(pHead1->val < pHead2->val){
                temp->next = pHead1;
                temp = pHead1;
                pHead1 = pHead1->next;
            }
            else{
                temp->next = pHead2;
                temp = pHead2;
                pHead2 = pHead2->next;
            }
        }
        // 将较长链表的剩余未遍历节点直接连接
        if(pHead1 == nullptr)
            temp->next = pHead2;
        else if(pHead2 == nullptr)
            temp->next = pHead1;
        return pHead;
    }
};