The penultimate K nodes in jz22 linked list

Topic link ： Last in the list k Nodes _ Niuke Tiba _ Cattle from

Be careful ：

1. Violent search ： First, find the length of the linked list len, If it is less than k, Return empty , Not less than k, go len-k No , The result is k Elements at and after the first position

2. Speed pointer ： The slow pointer points to the head node , Let's go first k Step , If it is longer than the length of the linked list, return null , If the pointer goes to the end , The slow pointer points to k Nodes .

Catalog

Method 1 : Violent search

Method 2 ： Speed pointer

Method 1 : Violent search

/**
 * struct ListNode {
 *	int val;
 *	struct ListNode *next;
 *	ListNode(int x) : val(x), next(nullptr) {}
 * };
 */
class Solution {
public:
    /**
     *  The class name in the code 、 Method name 、 The parameter name has been specified , Do not modify , Return the value specified by the method directly 
     *
     * 
     * @param pHead ListNode class  
     * @param k int integer  
     * @return ListNode class 
     */
    ListNode* FindKthToTail(ListNode* pHead, int k) {
        int len = 0;
        ListNode *p = pHead;
        while(p){
            len++;
            p = p->next;
        }
        if(len < k) return NULL;
        p = pHead;
        for(int i = 0 ; i < len-k; ++i){
            p = p->next;
        }
        return p;
    }
};

Method 2 ： Speed pointer

/**
 * struct ListNode {
 *	int val;
 *	struct ListNode *next;
 *	ListNode(int x) : val(x), next(nullptr) {}
 * };
 */
class Solution {
public:
    /**
     *  The class name in the code 、 Method name 、 The parameter name has been specified , Do not modify , Return the value specified by the method directly 
     *
     * 
     * @param pHead ListNode class  
     * @param k int integer  
     * @return ListNode class 
     */
    ListNode* FindKthToTail(ListNode* pHead, int k) {
        ListNode* fast = pHead ,*slow = pHead;
        // Let's go first k Step 
        for(int i = 0; i < k; ++i){
            if(fast != nullptr){
                fast = fast->next;   
            }else{
                return slow = nullptr;
            }
        }
        while(fast){
            fast = fast->next;
            slow = slow->next;
        }
         return slow;
    }
};