使用二级指针完成链表操作

最新推荐文章于 2024-11-19 07:49:11 发布

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本文介绍了一种链表操作算法，该算法能够根据给定的阈值x，将链表中所有节点按值分为两部分：一部分节点值小于x，另一部分节点值大于等于x，并保持原有的相对顺序。通过使用二级指针技巧，该算法能优雅地解决这一问题，适用于大多数链表结构问题。

//=============================QUESTION =================================
// Rearrange a list in such a way: given x, all nodes with val >= x should come after those with val < x.
// for example, given x = 5, a list {0 , 3 , 6 , 2 , 5} should be transformed into : { 0 , 3, 2, 6 , 5}. Of course, {0 , 3 , 2 , 5 ,6 } also meets the requirement.
// =====================================================================

#include <iostream>

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

//***** using second-order pointer can solve this question beautifully.  ****
//in fact, second-order pointer can handle almost all list structure problems neatly.
ListNode* rearrange(ListNode* pHead, int x)
{
    ListNode* smaller = NULL, **smaller_to_fill = &smaller;
    ListNode* bigger = NULL, **bigger_to_fill = &bigger;
    while(pHead)
    {
        if (pHead->val >= x)
        {
            *bigger_to_fill = pHead;
            bigger_to_fill = &(pHead->next);
        }
        else
        {
            *smaller_to_fill = pHead;
            smaller_to_fill = &(pHead->next);
        }
        pHead = pHead->next;
    }
    *smaller_to_fill = bigger;
    *bigger_to_fill = NULL;
    return smaller;
}





////TEST 

using namespace std;

int main(int argc, const char * argv[]) {

    ListNode* head, **p_to_fill = &head;

    //create a linklist
    for (int i = 0; i < 30; i++) {
        ListNode* new_node = new ListNode( (i * 101) % 31);
        *p_to_fill = new_node;
        p_to_fill = &(new_node->next);
    }
    
    //before rearrangement
    ListNode* p = head;
    while(p)
    {
        cout << p->val << "  ";
        p = p->next;
    }
    
    
    cout << "\n\n\n\n";
    
    ListNode* p_rearranged = rearrange(head, 15);
    
    //after rearrangement
    p = p_rearranged;
    while(p)
    {
        cout << p->val << "  ";
        p = p->next;
    }

    //delete the list
    ListNode** p_to_delete = &head;
    while(*p_to_delete)
    {
        ListNode* node = *p_to_delete;
        p_to_delete = &(node->next);
        delete node;
    }

    return 0;
}