2.4链表分割

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};*/
class Partition {
public:
    ListNode* partition(ListNode* pHead, int x) {
        // write code here
        ListNode* pFirst;
        ListNode* pHead1 = NULL;
        ListNode* pHead2 = NULL;
        ListNode* pBig = NULL;
        ListNode* pSmall = NULL;
        pFirst = pHead;
        for(pFirst; pFirst != NULL; pFirst = pFirst->next) {         
                if((pFirst->val) < x) { 
                    if(pSmall == NULL) {
                        pSmall = pFirst;
                        //pSmall->val = pFirst->val;//值不需要赋给，直接存有的
                        pHead1 = pSmall;
                    }
                    else{
                        pSmall->next = pFirst;
                        //pSmall->next->val = pFirst->val;
                        pSmall = pFirst; //pSmall指向当前的pFirst，以便进行下一次循环
                    }
                }     
                else{  
                    if(pBig == NULL) {
                        pBig = pFirst;
                        //pBig->val = pFirst->val;
                        pHead2 = pBig;
                    }
                    else{
                        pBig->next = pFirst;
                    //pBig->next->val = pFirst->val;
                        pBig = pFirst;
                    } 
                }
                  
        }
        if(pSmall && pBig) {pBig->next = NULL; pSmall->next = pHead2; return pHead1;}
        else if(pSmall) {pSmall->next = NULL; return pHead1;}
            else {pBig->next = NULL; return pHead2;}
        
        ////写法1：
        //if(pBig) {pBig->next = NULL;} //需要将后面的给去掉
        //if(pSmall){ pSmall->next = pHead2;return pHead1;}
        //else return pHead2;
        
        ////写法2：
        //if(pBig != NULL) pBig->next = NULL;
        //if(pSmall != NULL) {pSmall->next = pHead2;}
        //else{ pHead1 = pHead2; } 
        //return pHead1;
       
    }
};

这道题被折磨死了，整了好久一直编译通不过出问题，原因出在最后返回值那里。

1.最后一个链表末尾指针需要置NULL,不然总是出现程序打印过多数据。

2.后面的几种情况需要进行判断，应该可以优化写法。