[牛客]链表分割

编写代码，以给定值x为基准将链表分割成两部分，所有小于x的结点排在大于或等于x的结点之前 。

牛客链接

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最简单思路

因为头插必然改变顺序,所以使用尾插

大于的尾插到一个链表

小于的尾插到一个链表

然后链接起来

使用哨兵位的头结点,防止太多问题的产生

/*
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
    struct ListNode* LessTail,* LessGuardHead,* greaterGuardHead,* greaterTail;     
    LessGuardHead = LessTail = (struct ListNode*)malloc(sizeof(struct ListNode));
    greaterGuardHead = greaterTail = (struct ListNode*)malloc(sizeof(struct ListNode));
    greaterTail->next = LessTail->next = NULL;

    struct ListNode* cur = pHead;
    while(cur)
        {
            if(cur->val < x)
            {
                LessTail->next = cur;
                LessTail = LessTail->next;
            }
            else 
            {
                greaterTail->next = cur;
                greaterTail = greaterTail->next;

            }
            cur = cur->next;
        }
    LessTail->next = greaterGuardHead->next;



    pHead = LessGuardHead->next;
    free(greaterGuardHead);
    free(LessGuardHead);
    return pHead;

}
};

内存超限一般是发生了死循环,而单链表中发生死循环,我们考虑环状链表,如下图两个链表组合后,最后一个结点还指向原来后面的结点,构成了环状链表,所以尾插后切记要置空!!

最终代码:

/*
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
    struct ListNode* LessTail,* LessGuardHead,* greaterGuardHead,* greaterTail;     
    LessGuardHead = LessTail = (struct ListNode*)malloc(sizeof(struct ListNode));
    greaterGuardHead = greaterTail = (struct ListNode*)malloc(sizeof(struct ListNode));
    greaterTail->next = LessTail->next = NULL;

    struct ListNode* cur = pHead;
    while(cur)
        {
            if(cur->val < x)
            {
                LessTail->next = cur;
                LessTail = LessTail->next;
            }
            else 
            {
                greaterTail->next = cur;
                greaterTail = greaterTail->next;

            }
            cur = cur->next;
        }
    LessTail->next = greaterGuardHead->next;

    greaterTail->next = NULL;//避免环状链表

    pHead = LessGuardHead->next;
    free(greaterGuardHead);
    free(LessGuardHead);
    return pHead;

}
};