Today I solved..

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LeetCode 75 - Level 1

Day 4. Linked List

• 876 Middle of the Linked List (Easy)

• 142 Linked List Cycle II (Medium)

  • O(N) time, O(N) space

    class Solution:
    def detectCycle(self, head: Optional[ListNode]) -> Optional[ListNode]:
        p = head
        memo = set()
        while p != None:
            if p not in memo:
                memo.add(p)
            else:
                break
            p = p.next
        return p
    

  • O(N) time, O(1) space, Floyd's tortois and hare algorithm (I watched youtube)

    class Solution:
    def detectCycle(self, head: Optional[ListNode]) -> Optional[ListNode]:
    
        if head == None or head.next == None:
            return None
    
        slow = fast = head
        while True:
            if fast.next == None or fast.next.next == None:
                return None
            slow = slow.next
            fast = fast.next.next
            if slow == fast:
                break
    
        while head != slow:
            head = head.next
            slow = slow.next
    
        return head
    

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Algorithm - Algorithm I

Day 4. Two Pointers

• 344 Reverse String (Easy)
• 557 Reverse Words in a String III (Medium)