[Lyla] Week 02 solutions

3sum/pmjuu.py

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+from typing import List
+
+class Solution:
+    def threeSum(self, nums: List[int]) -> List[List[int]]:
+        result = []
+        nums.sort() # sort nums before using two-pointers
+
+        for i, num in enumerate(nums):
+            # skip duplicated targets
+            if i > 0 and nums[i] == nums[i - 1]:
+                continue
+
+            target = -num
+            left, right = i + 1, len(nums) - 1
+
+            while left < right:
+                if nums[left] + nums[right] == target:
+                    result.append([num, nums[left], nums[right]])
+
+                    # skip duplicated numbers ( ex. nums = [-3 0 0 0 3 3] )
+                    while left < right and nums[left] == nums[left + 1]:
+                        left += 1
+                    while left < right and nums[right] == nums[right -1]:
+                        right -= 1
+
+                    left += 1
+                    right -= 1
+                elif nums[left] + nums[right] < target:
+                    left += 1
+                else:
+                    right -= 1
+
+        return result
+
+
+# Time Complexity: O(n^2)
+# - Sorting takes O(n log n).
+# - The outer loop runs O(n) times, and the two-pointer approach inside runs O(n) for each iteration.
+# - Combined, the overall time complexity is O(n^2).
+
+# Space Complexity: O(k)
+# - The result list uses O(k) space, where k is the number of unique triplets in the output.

climbing-stairs/pmjuu.py

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+class Solution:
+    def climbStairs(self, n: int) -> int:
+        # dp[i] represents the number of distinct ways to climb to the ith step.
+        # Base cases:
+        # - There is 1 way to reach step 0 (doing nothing).
+        # - There is 1 way to reach step 1 (a single step).
+        dp = [0] * (n + 1)
+        dp[0], dp[1] = 1, 1
+
+        for i in range(2, n + 1):
+            dp[i] = dp[i - 1] + dp[i - 2]
+
+        return dp[n]
+
+# Complexity
+# - time: O(n)
+# - space: O(n)
+
+class Solution:
+    def climbStairs(self, n: int) -> int:
+        prev, curr = 1, 1
+
+        for _ in range(2, n + 1):
+            prev, curr = curr, prev + curr
+
+        return curr
+
+# Complexity
+# - time: O(n)
+# - space: O(1)

construct-binary-tree-from-preorder-and-inorder-traversal/pmjuu.py

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+from typing import List, 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 buildTree(self, preorder: List[int], inorder: List[int]) -> Optional[TreeNode]:
+        inorder_map = {value: idx for idx, value in enumerate(inorder)}
+        self.preorder_idx = 0
+
+        def helper(left: int, right: int) -> Optional[TreeNode]:
+            if left > right:
+                return None
+
+            root_val = preorder[self.preorder_idx]
+            self.preorder_idx += 1
+
+            root = TreeNode(root_val)
+            root.left = helper(left, inorder_map[root_val] - 1)
+            root.right = helper(inorder_map[root_val] + 1, right)
+
+            return root
+
+        return helper(0, len(inorder) - 1)
+
+
+# Time Complexity: O(n)
+# - Each node is visited exactly once in preorder, and the dictionary lookup in inorder_map takes O(1) per node.
+
+# Space Complexity: O(n)
+# - The hash map (inorder_map) uses O(n) space.
+# - The recursion stack uses up to O(h) space, where h is the height of the tree (O(n) in the worst case, O(log n) for a balanced tree).

decode-ways/pmjuu.py

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+class Solution:
+    def numDecodings(self, s: str) -> int:
+        if s[0] == "0":
+            return 0
+
+        prev, curr = 1, 1
+
+        for i in range(1, len(s)):
+            temp = curr
+
+            if s[i] == "0":
+                if s[i - 1] in ("1", "2"):
+                    curr = prev
+                else:
+                    return 0
+            else:
+                two_num = int(s[i - 1] + s[i])
+                is_two_num_decoded = 10 <= two_num <= 26
+                if is_two_num_decoded:
+                    curr += prev
+
+            prev = temp
+
+        return curr
+
+
+# Time Complexity: O(n)
+# - The loop iterates through the string once, where n is the length of the string.
+
+# Space Complexity: O(1)
+# - Only two variables (prev and curr) are used, independent of the input size.

valid-anagram/pmjuu.py

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+from collections import Counter
+
+class Solution:
+    def isAnagram(self, s: str, t: str) -> bool:
+        return Counter(s) == Counter(t)
+
+# Python의 collections.Counter는 유니코드 문자를 지원합니다.
+# 시간복잡도: O(n)
+    # Counter는 해시 테이블 기반의 연산으로 동작하며, 각 문자열의 문자를 한 번씩 순회합니다.
+# 공간복잡도: O(n)
+    # 각 문자열에 대해 Counter 객체를 생성하므로, 최악의 경우 각 문자마다 빈도를 저장하는 데 O(n) 크기의 해시테이블이 필요합니다.