[yyyyyyyyyKim] WEEK 02 solutions

3sum/yyyyyyyyyKim.py

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+class Solution:
+    def threeSum(self, nums: List[int]) -> List[List[int]]:
+
+        answer = []
+
+        # 정렬
+        nums.sort()
+
+        for i in range(len(nums)-2):
+            # i 중복제거
+            if i > 0 and nums[i] == nums[i - 1]:
+                continue
+
+            left = i+1
+            right = len(nums)-1
+
+            while left < right:
+                if nums[i] + nums[left] + nums[right] == 0:
+                    answer.append([nums[i],nums[left],nums[right]])
+
+                    # left 중복제거
+                    while left < right and nums[left] == nums[left+1]:
+                        left += 1
+                    # right 중복제거
+                    while left < right and nums[right] == nums[right-1]:
+                        right -= 1
+
+                    left += 1
+                    right -= 1
+
+                elif nums[i] + nums[left] + nums[right] < 0:
+                    left += 1
+                else:
+                    right -= 1
+
+        return answer

climbing-stairs/yyyyyyyyyKim.py

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+class Solution:
+    def climbStairs(self, n: int) -> int:
+
+        if n <= 3:
+            return n
+
+        a, b = 1, 2
+
+        for i in range(3,n+1):
+            a, b = b, a+b
+
+        return b

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

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+class Solution:
+    def productExceptSelf(self, nums: List[int]) -> List[int]:
+
+        answer = [1]*len(nums)
+
+        left = 1
+        for i in range(len(nums)):
+            answer[i] *= left
+            left *= nums[i]
+
+        right = 1
+        for i in range(len(nums)-1,-1,-1):
+            answer[i] *= right
+            right *= nums[i]
+
+        return answer

valid-anagram/yyyyyyyyyKim.py

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+class Solution:
+    def isAnagram(self, s: str, t: str) -> bool:
+
+        if sorted(s) == sorted(t):
+            return True
+        return False

validate-binary-search-tree/yyyyyyyyyKim.py

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+# 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:
+
+        # 깊이우선탐색(dfs), 재귀
+        def dfs(node, min_val, max_val):
+            if not node:
+                return True
+            if not (min_val < node.val < max_val):
+                return False
+            return (dfs(node.left, min_val, node.val) and
+                    dfs(node.right, node.val, max_val))
+
+        return dfs(root, float('-inf'), float('inf'))