剑指offer之链表环入口

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/*
 public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}
*/

/**
求链表环入口：
1. 确定有无环
2. 确定环的长度
3. 快慢指针确定入口
**/
public class Solution {

    public ListNode EntryNodeOfLoop(ListNode pHead)
    {
        if(pHead == null)
            return null;
        else if(pHead.next == null)
            return null;
        
        ListNode slow = pHead;
        ListNode fast = pHead;
        ListNode common = null;
        
        while(slow!=null && fast!=null)
        {
            slow = slow.next;
            fast = fast.next.next;
            
            if(slow == fast)
            {
                common = slow;
                break;
            }
        }
        
        if(common == null)
            return null;
        
        int len =1;
        ListNode start  = common.next;
        while(start!=common)
        {
            start = start.next;
            len++;
        }
        
        slow = pHead;
        fast = pHead;
        for(int i=0;i<len;i++)
        {
            fast = fast.next;
        }
        
        while(slow != fast)
        {
            slow = slow.next;
            fast = fast.next;
        }
        
        return slow;
    }
}