Penultimate k nodes linked list: prove safety offer 15

Title Description

Input a linked list, the linked list output reciprocal k-th node.

Problem-solving ideas

Speed ​​method using the pointer, so go fast pointer k steps, then let go slow start pointer, when the pointer reaches the quick list tail pointer just slow to reach the penultimate k nodes.

C ++ code to achieve:

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
            val(x), next(NULL) {
    }
};*/
class Solution {
public:
    ListNode* FindKthToTail(ListNode* pListHead, unsigned int k) {
　　　　
        if(pListHead==NULL){
            return NULL;
        }
        ListNode *p1,*p2;
        p1=pListHead;
        int count=0;
        int pos=k;
        while(POS> 0 && P1 =! NULL) { 
           P1 = P1-> Next; 
           POS - ; 
           COUNT ++ ; 
        } 
        // if the chain length is less than K, inputs are not valid, return NULL 
        IF (COUNT =! K) {
             return NULL; 
        } 
        P2 = pListHead;
         the while (P1 =! NULL) { 
           P1 = P1-> Next; 
           P2 = P2-> Next; 
        } 
        return P2; 
    } 
};