Leetcode 160

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

class Solution(object):
    
    def getIntersectionNode(self, headA, headB):
        """
        :type head1, head1: ListNode
        :rtype: ListNode
        """
        if not(headA and headB):
            return None
        
        p = headA
        q = headB
        
        lenA = 0
        lenB = 0
        
        while p.next:
            p = p.next
            lenA += 1
        while q.next:
            q = q.next
            lenB += 1
        
        if p.val != q.val:
            return None
        else:
            # intersection
            p = headA
            q = headB
            while (lenA > lenB):
                p = p.next
                lenA -= 1
            while (lenB > lenA):
                q = q.next
                lenB -= 1
                
            while p.val != q.val or p.next != q.next:
                p = p.next
                q = q.next
                
            return p