[LeetCode] Challenge log 160

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160. Intersection of Two Linked Lists

Algorithm1:

  1. traverse both lists and count len(a) and len(b)
  2. chop |len(b) - len(a)| nodes from the head of the longer list
  3. travese both lists simutaneously until two pointers hit

Algorithm2:

  1. let A = A+B, B = B+A such that two lists are alighn by their tails
  2. travese both lists simutaneously until two pointers hit

Analysis:

Both algs align the two lists by their tails.

Write a program to find the node at which the intersection of two singly linked lists begins.

For example, the following two linked lists:

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A:          a1 → a2
                   ↘
                     c1 → c2 → c3
                   ↗            
B:     b1 → b2 → b3

begin to intersect at node c1.

Notes:

  • If the two linked lists have no intersection at all, return null.
  • The linked lists must retain their original structure after the function returns.
  • You may assume there are no cycles anywhere in the entire linked structure.
  • Your code should preferably run in O(n) time and use only O(1) memory.
Soulution:
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# alg 1
class Solution(object):
    def getIntersectionNode(self, headA, headB):
        """
        :type head1, head1: ListNode
        :rtype: ListNode
        """
        # measure len
        cur = headA
        lenA = 0
        while cur:
            lenA += 1
            cur = cur.next
            
        cur = headB
        lenB = 0
        while cur:
            lenB += 1
            cur = cur.next
        
        # make 2 the same len
        if lenA > lenB:
            while lenA > lenB:
                headA = headA.next
                lenA -= 1
        else:
            while lenB > lenA:
                headB = headB.next
                lenB -= 1
        
        # check one by one:
        while headA:
            if headA == headB:
                return headA
            else:
                headA = headA.next
                headB = headB.next
        
        return None
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# alg 2
class Solution(object):
    def getIntersectionNode(self, headA, headB):
        """
        :type head1, head1: ListNode
        :rtype: ListNode
        """
        if not headA or not headB: return None
        pa = headA
        pb = headB
        while pa != pb: # either hit or end up both None
            if not pa: 
                pa = headB 
            else: pa = pa.next
            if not pb: 
                pb = headA 
            else: pb = pb.next            

        return pa
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