剑指Offer：链表中环的入口点（Python）

题目

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。

思路

把链表中节点值放到dict数组中，并记录出现的次数，如果出现次数超过一次，则为环的入口节点。

解答

方法一

class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None
class Solution:
    def EntryNodeOfLoop(self, pHead):
        if pHead is None:
            return None
        num, dict, flag = [], {}, True
        tempans = 0
        while pHead and flag == True:
            num.append(pHead.val)
            numset = set(num)
            for c in numset:
                dict[c] = 0
            for c in num:
                dict[c] += 1
            for c in num:
                if dict[c] > 1:
                    flag = False
                    tempans = c
            pHead = pHead.next
        while pHead:
            if pHead.val == tempans:
                return pHead
            pHead = pHead.next
        return None

思路

快慢指针

方法二

class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None
class Solution:
    def EntryNodeOfLoop(self, pHead):
        if pHead == None:
            return None
        p = pHead
        q = pHead
        while p and p.next:
            p = p.next.next
            q = q.next
            if p == q:
                break
        if p == None or p.next == None:
            return None
        p = pHead
        while p != q:
            p = p.next
            q = q.next
        return p