[leetcode]linked-list-cycle-ii

思路

1. 沿著 [linked-list-cycle]的思路， 但是返回的node* 是指向自己的。List也都被破壞
2. 使用runner

---

Solution 1 （未通過， Time Limit Exceeded）

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        if(head==nullptr){
            return head;
        }

        while(head!=nullptr){
            if(head->next==head){
                return head;
            }
            ListNode* nextNode;
            nextNode = head->next;
            head->next = head;
            head = nextNode;
        }
        return head;
    }
};

---

Solution 2

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        //start from the head
        ListNode* walker = head;
        ListNode* runner = head;

        //walker and runner, run!
        int n=0;
        while(true){
            if(walker&&runner){
                //walker goed 1 step a time
                walker = walker->next;
                //runner goes 2 steps a time
                if(runner->next){ 
                    runner = runner->next->next;
                }else{
                    n = -1;
                    break;
                }
                n++;
            }else{ //one of them get to the end(actually, runner of course), there`s no cycle in the list
                n = -1;
                break;
            }

            //runner and walker meet
            if(runner==walker){ 
                break;
            }
        }

        //we can proof that, when the runner and walker meet, the variable `n` will be the length 
        //of the cycle , and if we let runner go n steps first and then let them go 1 step each time, they
        //will meet at the start of the cycle.

        if(n==-1){ //no cycle
            return nullptr;
        }else{
            walker = head;
            runner = head;
            //let runner go n steps first
            for(int i=0;i<n;i++){
                runner = runner->next;
            }
            //when runner meets walker, the meet point will be the start of the cycle
            while(runner!=walker){
                walker = walker->next;
                runner = runner->next;
            }
            return walker;
        }
    }
};