[sangbeenmoon] WEEK 03 solutions

combination-sum/sangbeenmoon.py

@@ -0,0 +1,20 @@
+# TC : O(2^target) 재귀 트리의 분기 수
+# SC : O(target/min(candidates)) 재귀 스택 깊이
+
+class Solution: 
+    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
+        self.answer = []
+        for i in range(len(candidates)):
+            self.go(i, candidates, [candidates[i]], candidates[i], target)
+
+        return self.answer
+
+    def go(self, i:int, candidates: List[int], nums: List[int], sum_: int, target: int):
+        if sum_ == target:
+            self.answer.append(nums.copy())
+
+        for j in range(i, len(candidates)):
+            if candidates[j] + sum_ <= target:
+                nums.append(candidates[j])
+                self.go(j, candidates, nums, sum_ + candidates[j], target)
+                nums.pop()

decode-ways/sangbeenmoon.py

@@ -0,0 +1,36 @@
+# idea
+# 마지막 두자리 수가 독립 가능 -> dp[i] = dp[i-1] + dp[i-2]
+#   마지막 한자리 수가 독립 가능 -> dp[i] = dp[i-1]
+#     위 두 케이스 모두 불가능 -> 0 반환
+# TC : O(n)
+# SC : O(n)
+
+class Solution:
+    def numDecodings(self, s: str) -> int:
+        dp = [1] * len(s)
+
+        if s[0] == '0':
+            return 0
+
+        if '1' <= s[0] and s[0] <= '9':
+            dp[0] = 1
+
+        for i in range(1, len(s)):
+            target = s[i-1:i+1]
+
+            if int(s[i]) != 0 and 10 <= int(target) and int(target) <= 26:
+                dp[i] = dp[i-1] + dp[i-2]
+                continue
+
+            if int(s[i]) == 0 and 10 <= int(target) and int(target) <= 26:
+               dp[i] = dp[i-2]
+               continue
+
+            target = s[i]
+            if 1 <= int(target) and int(target) <= 9:
+                dp[i] = dp[i-1]
+                continue
+
+            return 0
+
+        return dp[len(s) - 1]

maximum-subarray/sangbeenmoon.py

@@ -0,0 +1,12 @@
+# 30분 내로 풀지 못함.
+# prefix sum 을 사용해서 풀어보려고 했으나 시간 초과.
+# dp 문제임을 파악할 수 있는 직관 필요...
+
+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], nums[i] + dp[i-1])
+
+        return max(dp)

number-of-1-bits/sangbeenmoon.py

@@ -0,0 +1,9 @@
+class Solution:
+    def hammingWeight(self, n: int) -> int:
+        cnt = 0
+        while n > 0:
+            if n % 2 == 1:
+                cnt = cnt + 1
+            n = n // 2
+            print(n)
+        return cnt

valid-palindrome/sangbeenmoon.py

@@ -0,0 +1,21 @@
+# TC : O(n) 
+# SC : O(n)
+
+class Solution:
+    def isPalindrome(self, s: str) -> bool:
+        refined_s = []
+
+        for ch in s:
+            if 'a' <= ch and ch <= 'z' or ('0' <= ch and ch <= '9'):
+                refined_s.append(ch)
+                continue
+
+            if 'A' <= ch and ch <= 'Z':
+                refined_s.append(ch.lower())
+
+        for i in range(len(refined_s)):
+            j = len(refined_s) - i - 1
+            if refined_s[i] != refined_s[j]:
+                return False
+
+        return True