[devyejin] WEEK 03 solutions

3sum/devyejin.py

@@ -0,0 +1,36 @@
+class Solution:
+    def threeSum(self, nums: List[int]) -> List[List[int]]:
+        nums.sort() # O(n)
+
+        n = len(nums)
+        result = []
+
+        for i in range(n - 2):
+            # target을 잡을때도 이전에 구했다면 패스
+            if i > 0 and nums[i] == nums[i - 1]:
+                continue
+
+            target = - nums[i]
+            left, right = i + 1, n - 1
+
+            while left < right:
+                two_sum = nums[left] + nums[right]
+
+                if two_sum == target:
+                    result.append([nums[i], nums[left], nums[right]])
+
+                    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 two_sum < target:
+                    left += 1
+                else:
+                    right -= 1
+
+        return result

climbing-stairs/devyejin.py

@@ -0,0 +1,17 @@
+class Solution(object):
+    def climbStairs(self, n):
+        """
+        :type n: int
+        :rtype: int
+        """
+        if n <= 2:
+            return n
+
+        dp = [0] * (n + 1)
+        dp[1], dp[2] = 1, 2
+
+        for i in range(3, n + 1):
+            dp[i] = dp[i-1] + dp[i-2]
+
+        return dp[n]
+

combination-sum/devyejin.py

@@ -0,0 +1,20 @@
+class Solution:
+    def combinationSum(self, candidates: list[int], target: int) -> list[list[int]]:
+
+        def backtracking(start, total, result):
+            if total == target:
+                answer.append(result[:])
+                return
+
+            if sum(result) > target:
+                return
+
+            for i in range(start, len(candidates)):
+                result.append(candidates[i])
+                backtracking(i, total + candidates[i], result)
+                result.pop()
+
+        answer = []
+        backtracking(0, 0, [])
+        return answer
+

decode-ways/devyejin..py

@@ -0,0 +1,24 @@
+class Solution:
+    def numDecodings(self, s: str) -> int:
+
+        if not s or s[0] == "0":
+            return 0
+
+        n = len(s)
+        dp = [0] * (n + 1)
+
+        dp[0] = 1
+        dp[1] = 1
+
+        for i in range(2, n + 1):
+            if s[i - 1] != "0":
+                dp[i] += dp[i - 1]
+
+            two_digit = int(s[i - 2: i])
+            if 10 <= two_digit <= 26:
+                dp[i] += dp[i - 2]
+
+        return dp[n]
+
+
+

maximum-subarray/devyejin.py

@@ -0,0 +1,10 @@
+class Solution:
+    def maxSubArray(self, nums: list[int]) -> int:
+        dp = [0] * len(nums)
+        dp[0] = nums[0]
+
+        for i in range(1, len(nums)):
+            dp[i] = max(nums[i], dp[i - 1] + nums[i])
+
+        return max(dp)
+

number-of-1-bits/devyejin.py

@@ -0,0 +1,7 @@
+class Solution:
+    def hammingWeight(self, n: int) -> int:
+        result = 0
+        while n:
+            result += n % 2
+            n //= 2
+        return result

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

@@ -0,0 +1,16 @@
+class Solution:
+    def productExceptSelf(self, nums):
+        n = len(nums)
+        result = [1] * n
+
+        left_product = 1
+        for i in range(n):
+            result[i] = left_product
+            left_product *= nums[i]
+
+        right_product = 1
+        for i in range(n - 1, -1, -1):
+            result[i] *= right_product
+            right_product *= nums[i]
+
+        return result

valid-palindrome/devyejin.py

@@ -0,0 +1,5 @@
+# isalnum() : 문자열이 영어, 한글 숫자 -> True , 아니라면 False
+class Solution:
+    def isPalindrome(self, s: str) -> bool:
+        changed_s = [c.lower() for c in s if c.isalnum()]
+        return changed_s == changed_s[::-1]

validate-binary-search-tree/devyejin.py

@@ -0,0 +1,51 @@
+# Definition for a binary tree node.
+from collections import deque
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+def list_to_tree(arr):
+    if not arr:
+        return None
+#      -> 레벨로 주고 있음
+    root = TreeNode(arr[0])
+    queue = deque([root]) # 큐에 root를 넣고 시작 queue = deque() queue.append(root)와 동일
+    i = 1
+
+    while queue and i < len(arr):
+        current = queue.popleft()
+
+        if i < len(arr) and arr[i] is not None:
+            current.left = TreeNode(arr[i])
+            queue.append(current.left)
+        i += 1
+
+        if i < len(arr) and arr[i] is not None:
+            current.right = TreeNode(arr[i])
+            queue.append(current.right)
+        i += 1
+
+    return root
+
+
+class Solution:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        self.prev = None
+        def inorder(node):
+            if not node:
+                return True
+
+            # 왼쪽 서브트리 확인
+            if not inorder(node.left):
+                return False
+
+            # 현재 노드 값 확인
+            if self.prev is not None and node.val <= self.prev:
+                return False
+            self.prev = node.val
+
+            return inorder(node.right)
+
+        return inorder(root)