链表中倒数第k个结点

问题

从1开始计数，计算倒数第k个结点的指针。例如：

思路

整着数到第k，然后前后一块往后走，前边的走到头，后边的极为倒数第k个结点，图示

注意

• 传入空指针
• k大于结点的个数

代码

ListNode* LastNNode(ListNode *root, int n)
{
    if (root == NULL || n <=0)
        return NULL;
    ListNode *cur = root;
    ListNode *pre = root;
    int cnt = 0;
    while (pre != NULL)
    {
        cnt++;
        if (cnt == n)
            break;
        pre = pre->next;
    }
    if (cnt < n)
    {
        return NULL;
    }
    else
    {
        cur = root;
        while (pre->next != NULL)
        {
            pre = pre->next;
            cur = cur->next;
        }
        return cur;
    }
}

执行

#include <iostream>
using namespace std;
struct ListNode
{
    int val;
    ListNode *next;
    ListNode(int v) : val(v), next(NULL) {}
};

ListNode* LastNNode(ListNode *root, int n)
{
    if (root == NULL || n <=0)
        return NULL;
    ListNode *cur = root;
    ListNode *pre = root;
    int cnt = 0;
    while (pre != NULL)
    {
        cnt++;
        if (cnt == n)
            break;
        pre = pre->next;
    }
    if (cnt < n)
    {
        return NULL;
    }
    else
    {
        cur = root;
        while (pre->next != NULL)
        {
            pre = pre->next;
            cur = cur->next;
        }
        return cur;
    }
}



ListNode* createList()
{
    ListNode *root = new ListNode(0);
    ListNode *p1 = new ListNode(1);
    ListNode *p2 = new ListNode(2);
    ListNode *p3 = new ListNode(3);
    root->next = p1;
    p1->next = p2;
    p2->next = p3;
    return root;
}

void deleteList(ListNode *root)
{
    ListNode *p = root;
    while(root != NULL)
    {
        p = root->next;
        delete(root);
        root = p;
    }
}

void tranverse(ListNode* root)
{
    while(root != NULL)
    {
        cout << root->val << " ";
        root = root->next;
    }
    cout << endl;
}

int main()
{

    ListNode *root = createList();
    root = NULL;

    ListNode *p = LastNNode(root, 1);
    cout << p << endl;
    if (p != NULL)
        cout << p->val << endl;
     
    deleteList(root);
}

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