[数据结构与算法]142. 环形链表 II

使用双指针，慢指针每次走一步，快指针走两步，如果快慢指针相遇，则有环。
若有环，设定速度相同的双指针，一个从头节点出发，一个从快慢指针相遇节点出发，相遇点就是环的入口
证明过程：https://github.com/youngyangyang04/leetcode-master/blob/master/problems/0142.%E7%8E%AF%E5%BD%A2%E9%93%BE%E8%A1%A8II.md

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def detectCycle(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        fast = head
        slow = head
        while (fast != None) and (fast.next != None):
            slow = slow.next
            fast = fast.next.next
            if slow == fast:
                curq = slow
                curp = head
                while (curp != curq):
                    curp = curp.next
                    curq = curq.next
                return curp
        
        return None