[sejineer] Week 03 solutions

combination-sum/sejineer.py

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+"""
+시간 복잡도: O(target * n)
+공간 복잡도: O(n)?
+개인적으로 어려웠던 문제라서 정답을 봤습니다.
+추후에 다시 복습할 예정입니다.
+"""
+class Solution:
+    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
+        dp = [[] for _ in range(target + 1)]
+        dp[0] = [[]]
+        for candidate in candidates:
+            for num in range(candidate, target + 1):
+                for combination in dp[num - candidate]:
+                    dp[num].append(combination + [candidate])
+        return dp[target]

decode-ways/sejineer.py

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+"""
+시간 복잡도: O(N)
+공간 복잡도: O(N)
+"""
+class Solution:
+    def numDecodings(self, s: str) -> int:
+        dp = [0] * len(s) + [1]
+
+        for i in reversed(range(len(s))):
+            if s[i] == "0":
+                dp[i] = 0
+            elif i + 1 < len(s) and int(s[i : i + 2]) < 27:
+                dp[i] = dp[i + 1] + dp[i + 2]
+            else:
+                dp[i] = dp[i + 1]
+
+        return dp[0]

maximum-subarray/sejineer.py

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+"""
+시간 복잡도 O(N)
+공간 복잡도 O(N)
+"""
+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/sejineer.py

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+"""
+시간 복잡도: O(logN)
+공간 복잡도: O(1)
+"""
+class Solution:
+    def hammingWeight(self, n: int) -> int:
+        result = 1
+        while n // 2 != 0:
+            a = n // 2
+            b = n % 2
+            result += b
+            n = a
+        return result

valid-palindrome/sejineer.py

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+"""
+시간 복잡도: O(N)
+공간 복잡도: O(N)
+"""
+class Solution:
+    def isPalindrome(self, s: str) -> bool:
+        filterd_s = [ch for ch in s.lower() if ch.isalnum()]
+        return filterd_s == filterd_s[::-1]