题目:通过有环链表，返回环路链表的开头结点.

核心代码:
<pre><code>` func findBeginingNode(headNode:ListNode) -> ListNode? {

    var slow:ListNode? = headNode
    var fast:ListNode? = headNode
    
    // 环路节点相交
    while fast != nil {
        slow = slow?.next
        fast = fast?.next?.next
        if slow?.value == fast?.value {
            break
        }
    }
    
    // 校验
    if fast == nil || fast?.next == nil {
        return nil
    }
    
    // 重置开始节点
    slow = headNode
    while slow?.value != fast?.value {
        slow = slow?.next
        fast = fast?.next
    }
    
    return fast
}`</code></pre>

测试代码:
<pre><code>var circleHead:ListNode = ListNode(value: "1") var circelNode1:ListNode = ListNode(value: "2") var circelNode2:ListNode = ListNode(value: "3") var circelNode3:ListNode = ListNode(value: "4") circleHead.next = circelNode1 circelNode1.next = circelNode2 circelNode2.next = circelNode3 circelNode3.next = circleHead var beginingNode:ListNode? = listNodeManger.findBeginingNode(headNode: circelNode1) print("FlyElephant---环路链表开始值---\(beginingNode!.value!)")</code></pre>