模拟

若是有一个公共节点x，那么后面的长度都一样，那么把长链表先走几步使得和短链表一样长，那么x前面的长度也一样了，一起走就到x了

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

class Solution(object):
    def findFirstCommonNode(self, headA, headB):
        """
        :type headA, headB: ListNode
        :rtype: ListNode
        """
        a, b = 0, 0
        da, db = headA, headB
        while da:
            a += 1
            da = da.next
        while db:
            b += 1
            db = db.next
        da, db = headA, headB
        while b > a:
            b -= 1
            db = db.next
        while a > b:
            a -= 1
            da = da.next
        while da and db:
            if da == db:
                return da
            da, db = da.next, db.next
        return None