Week 5 Solutions

Author: hoyeongkwak

best-time-to-buy-and-sell-stock/hoyeongkwak.py

@@ -0,0 +1,29 @@
+'''
+time complexity : O(n)
+space complexity : O(2n)
+
+Algorithm : dp
+주식 보유하는 경우:
+- 이전에 이미 보유 : hold[i - 1]
+- i번째 날 새로 사는 경우 : -prices[i]
+
+주식을 팔았을 떄:
+- 이전에 이미 팜 : sold[i - 1]
+- i번째 날 파는 경우 : hold[i-1] + prices[i]
+'''
+
+class Solution:
+    def maxProfit(self, prices: List[int]) -> int:
+        if not prices or len(prices) < 2:
+            return 0
+        n = len(prices)
+        hold = [0] * n
+        sold = [0] * n
+
+        hold[0] = -prices[0]
+        sold[0] = 0
+
+        for i in range(1, n):
+            hold[i] = max(hold[i - 1], -prices[i])
+            sold[i] = max(sold[i - 1], hold[i - 1] + prices[i])
+        return sold[n - 1]

group-anagrams/hoyeongkwak.py

@@ -0,0 +1,20 @@
+'''
+m : 문자열 길이
+n : 반복
+time complexity : O(m * n * log m)
+space complexity : O(m * n)
+algorithm : sort, hash, 
+각 문자열에 대해서 정렬을 하고, 정렬을 했을 때 동일한 문자에 대해서 hash table에 정렬 전 문자열을 추가
+그리고 마지막에 list 형태로 hash table의 value에 대해서 돌려 준다
+nat -> ant
+tan -> ant
+'''
+class Solution:
+    def groupAnagrams(self, strs: List[str]) -> List[List[str]]:
+        strResult = {}
+        for str in strs:
+            sortedStr = ''.join(sorted(str))
+            if sortedStr not in strResult:
+                strResult[sortedStr] = []
+            strResult[sortedStr].append(str)
+        return list(strResult.values())

implement-trie-prefix-tree/hoyeongkwak.py

@@ -0,0 +1,53 @@
+'''
+insert
+time complexity : O(m)
+space complexity : O(m)
+
+search
+time complexity : O(m)
+space complexity : O(1)
+
+startWith
+time complexity : O(m)
+space complexity : O(1)
+'''
+
+class TriedNode:
+    def __init__(self):
+        self.children = {}
+        self.isEnd = False
+    
+class Trie:
+    def __init__(self):
+        self.root = TriedNode()
+
+    def insert(self, word: str) -> None:
+        node = self.root
+        for char in word:
+            if char not in node.children:
+                node.children[char] = TriedNode()
+            node = node.children[char]
+        node.isEnd = True
+        
+
+    def search(self, word: str) -> bool:
+        node = self.root
+        for char in word:
+            if char not in node.children:
+                return False
+            node = node.children[char]
+        return node.isEnd
+
+    def startsWith(self, prefix: str) -> bool:
+        node = self.root
+        for char in prefix:
+            if char not in node.children:
+                return False
+            node = node.children[char]
+        return True
+
+# Your Trie object will be instantiated and called as such:
+# obj = Trie()
+# obj.insert(word)
+# param_2 = obj.search(word)
+# param_3 = obj.startsWith(prefix)

word-break/hoyeongkwak.py

@@ -0,0 +1,56 @@
+'''
+TriedNode
+a : alphabet size
+time complexity : O(m)
+space complexity : O(m * a)
+
+dp
+time complexity : O(n^2)
+space complexity : O(n)
+'''
+class TriedNode:
+    def __init__(self):
+        self.children = {}
+        self.isEnd = False
+class Trie:
+    def __init__(self):
+        self.root = TriedNode()
+    
+    def insert(self, word):
+        node = self.root
+        for char in word:
+            if char not in node.children:
+                node.children[char] = TriedNode()
+            node = node.children[char]
+        node.isEnd = True
+    
+    def search(self, s, start_idx):
+        node = self.root
+        endPosition = []
+
+        for i in range(start_idx, len(s)):
+            char = s[i]
+            if char not in node.children:
+                break
+            node = node.children[char]
+            if node.isEnd:
+                endPosition.append(i + 1)
+        return endPosition
+
+class Solution:
+    def wordBreak(self, s: str, wordDict: List[str]) -> bool:
+        trie = Trie()
+        for word in wordDict:
+            trie.insert(word)
+        n = len(s)
+        dp = [False] * (n + 1)
+        dp[0] = True
+
+        for i in range(n):
+            if not dp[i]:
+                continue
+            endPositions = trie.search(s, i)
+
+            for endPos in endPositions:
+                dp[endPos] = True
+        return dp[n]