题目：输入两个链表，找出它们的第一个公共节点.
核心代码:
<pre><code>`

func findFirstCommon(firstHead:ListNode?,nextHead:ListNode?) -> ListNode? {
    if firstHead == nil || nextHead == nil {
        return nil
    }
    var commonNode:ListNode?
    
    let firstlen:Int = listNodeLength(node: firstHead)
    let nextlen:Int = listNodeLength(node: nextHead)
    
    var firstTempNode:ListNode? = firstHead
    var nextTempNode:ListNode? = nextHead
    
    let gap:Int = abs(nextlen - firstlen)
    var count:Int = 0
    if firstlen > nextlen {
        while firstTempNode != nil && count < gap {
            count += 1
            firstTempNode = firstTempNode?.next
        }
    } else {
        while nextTempNode != nil && count < gap {
            count += 1
            nextTempNode = nextTempNode?.next
        }
    }
    
    while firstTempNode != nil && nextTempNode != nil {
        let value1:Int = firstTempNode!.value!
        let value2:Int = nextTempNode!.value!
        if value1 == value2 {
            commonNode = firstTempNode
            break
        }
        firstTempNode = firstTempNode?.next
        nextTempNode = nextTempNode?.next
    }
    
    return commonNode
}

func listNodeLength(node:ListNode?) -> Int {
    if node == nil {
        return 0
    }
    var count:Int = 0
    var tempNode:ListNode? = node
    while tempNode != nil {
        count += 1
        tempNode = tempNode?.next
    }
    return count
}`</code></pre>

测试代码:
<pre><code>`

var firstSearchNode:ListNode?
var firstSearchHead:ListNode?
searchNode.createList(&firstSearchNode, data: 1)
firstSearchHead = firstSearchNode

searchNode.createList(&firstSearchNode, data: 2)
searchNode.createList(&firstSearchNode, data: 3)
searchNode.createList(&firstSearchNode, data: 6)
searchNode.createList(&firstSearchNode, data: 7)

var nextSearchNode:ListNode?
var nextSearchHead:ListNode?
searchNode.createList(&nextSearchNode, data: 4)
nextSearchHead = nextSearchNode

searchNode.createList(&nextSearchNode, data: 5)
searchNode.createList(&nextSearchNode, data: 6)
searchNode.createList(&nextSearchNode, data: 7)

searchNode.printList(nextSearchHead)

var commonNode:ListNode? = searchNode.findFirstCommon(firstHead: firstSearchHead, nextHead: nextSearchHead)

if commonNode == nil {
print("没有公共节点")
} else {
print("第一个公共节点的值-(commonNode!.value!)")
}`</code></pre>