[doh6077] WEEK 02 solutions

3sum/doh6077.py

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+from typing import List
+
+class Solution:
+    def threeSum(self, nums: List[int]) -> List[List[int]]:
+        nums.sort()
+        n = len(nums)
+        res: List[List[int]] = []
+
+        for i in range(n - 2):
+            # i 중복 스킵
+            if i > 0 and nums[i] == nums[i - 1]:
+                continue
+
+            # nums[i] 이후는 전부 양수 → 더 이상 0 못 만듦
+            if nums[i] > 0:
+                break
+
+            left, right = i + 1, n - 1
+
+            while left < right:
+                total = nums[i] + nums[left] + nums[right]
+
+                if total == 0:
+                    res.append([nums[i], nums[left], nums[right]])
+
+                    left += 1
+                    right -= 1
+
+                    # left 중복 스킵
+                    while left < right and nums[left] == nums[left - 1]:
+                        left += 1
+
+                    # right 중복 스킵
+                    while left < right and nums[right] == nums[right + 1]:
+                        right -= 1
+
+                elif total < 0:
+                    left += 1
+                else:
+                    right -= 1
+
+        return res

top-k-frequent-elements/doh6077.py

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+class Solution:
+    # dictionary use 
+    def topKFrequent(self, nums: List[int], k: int) -> List[int]:
+        result = {}  # key: 원소, value: 등장 횟수
+        for n in nums:
+            if n in result:
+                result[n] = result[n] + 1
+            else:
+                result[n] = 1
+
+        # 가장 자주 등장한 원소 k개 반환
+        return sorted(result.keys(), key=lambda x: result[x], reverse=True)[:k]

validate-binary-search-tree/doh6077.py

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+class Solution:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        def validate(node: Optional[TreeNode], low: float, high: float) -> bool:
+            if not node:
+                return True
+            if not (low < node.val < high):
+                return False
+            return (validate(node.left, low, node.val) and
+                    validate(node.right, node.val, high))
+        return validate(root, float('-inf'), float('inf'))