[hellojoyworldz] WEEK 02 solutions

3sum/hellojoyworldz.py

@@ -0,0 +1,36 @@
+# 문제: https://leetcode.com/problems/3sum/
+# 해설: https://www.algodale.com/problems/3sum/
+# 위치: https://github.com/DaleStudy/leetcode-study/tree/main/3sum
+
+from typing import List
+
+
+class Solution:
+    def threeSum(self, nums: List[int]) -> List[List[int]]:
+        nums.sort()
+        result = []
+
+        for i in range(len(nums)):
+            if i > 0 and nums[i] == nums[i - 1]:
+                continue
+
+            left, right = i + 1, len(nums) - 1
+            while left < right:
+                total = nums[i] + nums[left] + nums[right]
+
+                if total < 0:
+                    left += 1
+                elif total > 0:
+                    right -= 1
+                else:
+                    result.append([nums[i], nums[left], nums[right]])
+                    left += 1
+                    right -= 1
+
+                    while left < right and nums[left] == nums[left - 1]:
+                        left += 1
+                    while left < right and nums[right] == nums[right + 1]:
+                        right -= 1
+
+        return result
+

climbing-stairs/hellojoyworldz.py

@@ -0,0 +1,11 @@
+# 문제: https://leetcode.com/problems/climbing-stairs/
+# 해설: https://www.algodale.com/problems/climbing-stairs/
+# 위치: https://github.com/DaleStudy/leetcode-study/tree/main/climbing-stairs
+
+class Solution:
+    def climbStairs(self, n: int) -> int:
+        prev2, prev1 = 1, 1
+        for _ in range(n - 1):
+            prev2, prev1 = prev1, prev2 + prev1
+        return prev1
+

product-of-array-except-self/hellojoyworldz.py

@@ -0,0 +1,24 @@
+# 문제: https://leetcode.com/problems/product-of-array-except-self/
+# 해설: https://www.algodale.com/problems/product-of-array-except-self/
+# 위치: https://github.com/DaleStudy/leetcode-study/tree/main/product-of-array-except-self
+
+from typing import List
+
+
+class Solution:
+    def productExceptSelf(self, nums: List[int]) -> List[int]:
+        n = len(nums)
+        answer = [1] * n
+
+        left = 1
+        for i in range(n):
+            answer[i] = left
+            left *= nums[i]
+
+        right = 1
+        for i in reversed(range(n)):
+            answer[i] *= right
+            right *= nums[i]
+
+        return answer
+

valid-anagram/hellojoyworldz.py

@@ -0,0 +1,8 @@
+# 문제: https://leetcode.com/problems/valid-anagram/
+# 해설: https://www.algodale.com/problems/valid-anagram/
+# 위치: https://github.com/DaleStudy/leetcode-study/tree/main/valid-anagram
+
+class Solution:
+    def isAnagram(self, s: str, t: str) -> bool:
+      return sorted(s) == sorted(t)
+

validate-binary-search-tree/hellojoyworldz.py

@@ -0,0 +1,26 @@
+# 문제: https://leetcode.com/problems/validate-binary-search-tree/
+# 해설: https://www.algodale.com/problems/validate-binary-search-tree/
+# 위치: https://github.com/DaleStudy/leetcode-study/tree/main/validate-binary-search-tree
+
+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 isValidBST(self, root: Optional[TreeNode]) -> bool:
+        def check(node: Optional[TreeNode], low: float, high: float) -> bool:
+            if not node:
+                return True
+
+            if not (low < node.val < high):
+                return False
+
+            return check(node.left, low, node.val) and check(node.right, node.val, high)
+
+        return check(root, float("-inf"), float("inf"))
+