Easy-to-understand python solution


  • 0
    Y
    class Solution(object):
    def reverseBetween(self, head, m, n):
        """
        :type head: ListNode
        :type m: int
        :type n: int
        :rtype: ListNode
        """
        if not head or not head.next:
            return head
    
        p = head
        beforeM = None
        for i in range(1,m):
            beforeM = p
            p = p.next
        atM = p # beforeM is the (m-1)th one in list before reverse, atM is the (m)th one.
    
        prev = p
        p = p.next
        for i in range(m+1, n+1): # reverse link from m to n
            tmp = p.next
            p.next = prev
            prev = p
            p = tmp
    
        atM.next = p # end of the sublist
        if beforeM:
            beforeM.next = prev
        else: # in case m == 1, i.e no one before m
            head = prev
        return head

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