[김병현, bhyun-kim] Solution for Week2 Assignment

invert-binary-tree/bhyun-kim.py

@@ -0,0 +1,40 @@
+"""
+226. Invert Binary Tree
+https://leetcode.com/problems/invert-binary-tree/description/
+
+Solution
+    Recursively swaps the left and right children of the root node.
+
+    1. Check if the root has left and right children.
+    2. If it does, invert the left and right children.
+    3. If it doesn't, invert the left or right child.
+    4. Return the root.
+
+Time complexity: O(n)
+Space complexity: O(n)
+
+"""
+from typing import Optional
+
+
+# Definition for a binary tree node.
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+
+class Solution:
+    def invertTree(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
+        has_left = hasattr(root, "left")
+        has_right = hasattr(root, "right")
+
+        if has_left and has_right:
+            root.left, root.right = self.invertTree(root.right), self.invertTree(root.left)
+        elif has_left:
+            root.left, root.right = None, self.invertTree(root.left)
+        elif has_right:
+            root.left, root.right = self.invertTree(root.right), None
+
+        return root

linked-list-cycle/bhyun-kim.py

@@ -0,0 +1,45 @@
+"""
+141. Linked List Cycle
+https://leetcode.com/problems/linked-list-cycle/description/
+
+Solution
+    Floyd's Tortoise and Hare algorithm:
+
+    1. Initialize two pointers, slower and faster, to the head of the linked list.
+        faster is one step ahead of slower.
+    2. Move the slower pointer by one and the faster pointer by two.
+    3. If the slower and faster pointers meet, return True.
+    4. If the faster pointer reaches the end of the linked list, return False.
+
+Time complexity: O(n)
+Space complexity: O(1)
+"""
+
+
+from typing import Optional
+
+
+# Definition for singly-linked list.
+class ListNode:
+    def __init__(self, x):
+        self.val = x
+        self.next = None
+
+
+class Solution:
+    def hasCycle(self, head: Optional[ListNode]) -> bool:
+        if not hasattr(head, "next"):
+            return False
+
+        slower = head
+        faster = head.next
+
+        while slower != faster:
+            if not hasattr(faster, "next"):
+                return False
+            elif not hasattr(faster.next, "next"):
+                return False
+            slower = slower.next
+            faster = faster.next.next
+
+        return True

merge-two-sorted-lists/bhyun-kim.py

@@ -0,0 +1,42 @@
+"""
+21. Merge Two Sorted Lists
+https://leetcode.com/problems/merge-two-sorted-lists/
+
+Solution
+    Recursively merges two sorted linked lists.
+
+    1. Check if either list is empty.
+    2. Compare the values of the two lists.
+    3. Return a new node with the smaller value and the merged list.
+
+Time complexity: O(n)
+Space complexity: O(n)
+"""
+from typing import Optional
+
+
+# Definition for singly-linked list.
+class ListNode:
+    def __init__(self, val=0, next=None):
+        self.val = val
+        self.next = next
+
+
+class Solution:
+    def mergeTwoLists(
+        self, list1: Optional[ListNode], list2: Optional[ListNode]
+    ) -> Optional[ListNode]:
+        if list1 is None:
+            return list2
+
+        if list2 is None:
+            return list1
+
+        if list1.val < list2.val:
+            val = list1.val
+            list1 = list1.next
+        else:
+            val = list2.val
+            list2 = list2.next
+
+        return ListNode(val=val, next=self.mergeTwoLists(list1, list2))

reverse-linked-list/bhyun-kim.py

@@ -0,0 +1,40 @@
+"""
+206. Reverse Linked List
+https://leetcode.com/problems/reverse-linked-list/description/
+
+Solution
+    Iteratively reverses a singly linked list.
+
+    1. Initialize two pointers, prev and curr, to None and the head of the linked list, respectively.
+    2. While curr is not None:
+        a. Save the next node of curr.
+        b. Set the next node of curr to prev.
+        c. Move prev and curr to the next nodes.
+
+Time complexity: O(n)
+Space complexity: O(1)
+
+"""
+
+
+from typing import Optional
+
+
+# Definition for singly-linked list.
+class ListNode:
+    def __init__(self, val=0, next=None):
+        self.val = val
+        self.next = next
+
+
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        prev, curr = None, head
+
+        while curr:
+            next_temp = curr.next
+            curr.next = prev
+            prev = curr
+            curr = next_temp
+
+        return prev

valid-parentheses/bhyun-kim.py

@@ -0,0 +1,40 @@
+"""
+20. Valid Parentheses
+https://leetcode.com/problems/valid-parentheses/description/
+
+Solution
+    Uses a stack to check if the parentheses are valid.
+
+    1. Initialize an empty stack.
+    2. Iterate through each character in the string.
+    3. If the character is an opening parenthesis, 
+        push it to the stack.
+    4. If the character is a closing parenthesis,
+        pop the stack and check if the opening parenthesis matches.
+    5. If the stack is empty, return True.
+    6. Otherwise, return False.
+
+Time complexity: O(n)
+Space complexity: O(n)
+"""
+
+
+class Solution:
+    def isValid(self, s: str) -> bool:
+        stack = ""
+        opening = ["(", "[", "{"]
+        closing = [")", "]", "}"]
+
+        for _s in s:
+            if _s in opening:
+                stack = _s + stack
+            else:
+                if stack:
+                    if opening[closing.index(_s)] == stack[0]:
+                        stack = stack[1:]
+                    else:
+                        return False
+                else:
+                    return False
+
+        return len(stack) == 0