剑指Offer 总结

给一个链表，若其中包含环，请找出该链表的环的入口结点，否则，输出null。

public class Solution {
    public ListNode EntryNodeOfLoop(ListNode pHead) {
        if(pHead == null) return null;
        ListNode f = pHead;
        ListNode s = pHead;
        //判断是否有环
        boolean hasLoop = false;
        while(s != null && f.next != null){
            f = f.next.next;
            s = s.next;
            if(f == s){
                hasLoop = true;
                break;
            }
        }
        if(!hasLoop) return null;
        //求得环的长度
        f = f.next.next;
        s = s.next;
        int length = 1;
        while(f != s){
            f = f.next.next;
            s = s.next;
            length++;
        }
        //找到环的入口
        f = s = pHead;
        for (int i = 0; i < length; i++)
            f = f.next;
        while (f != s){
            f = f.next;
            s = s.next;
        }
        return f;
    }