[njngwn] WEEK 05 Solutions

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

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+class Solution:
+    # Time Complexity: O(n), n: len(prices)
+    # Space Complexity: O(1)
+    def maxProfit(self, prices: List[int]) -> int:
+        min_price = prices[0]
+        profit = 0
+
+        for i in range(1, len(prices)):
+            min_price = min(min_price, prices[i])
+            profit = max(profit, prices[i] - min_price)
+
+        return profit

climbing-stairs/njngwn.py

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+class Solution:
+    # dynamic programming with bottom-up
+    # Time Complexity: O(n)
+    # Space Complexity: O(1)
+    def climbStairs(self, n: int) -> int:
+        if n <= 2: return n
+        prev1, prev2 = 2, 1
+
+        for i in range(3, n+1):
+            prev1, prev2 = prev2 + prev1, prev1
+
+        return prev1

encode-and-decode-strings/njngwn.py

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+class Solution:
+
+    def encode(self, strs: List[str]) -> str:
+        encoded = ''
+
+        for s in strs:
+            encoded += str(len(s)) + '#' + s
+
+        return encoded
+
+    def decode(self, s: str) -> List[str]:
+        decoded = []
+
+        i = 0
+        while i < len(s):
+            j = i  # j is a pointer for beginning of string
+            # find '#' in string
+            while s[j] != '#':
+                j += 1
+
+            length = int(s[i:j])
+            word = s[j + 1:j + length + 1]
+            decoded.append(word)
+            i = j + length + 1
+
+        return decoded

group-anagrams/njngwn.py

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+class Solution:
+    # Time Complexity: O(n), n: len(strs)
+    # Space Complexity: O(n), n: len(strs)
+    def groupAnagrams(self, strs: List[str]) -> List[List[str]]:
+        anagram_groups = dict()  # key: string in an alphabetical order, values: strings
+
+        for s in strs:
+            word = ''.join(sorted(s))
+            if word in anagram_groups:
+                anagram_groups[word].extend([s])
+            else:
+                anagram_groups[word] = [s]
+
+        return list(anagram_groups.values())

valid-anagram/njngwn.py

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+from collections import Counter
+
+class Solution:
+    # Time Complexity: O(n), n: max(len(s), len(t))
+    # Space Complexity: O(k), k: number of letters
+    def isAnagram(self, s: str, t: str) -> bool:
+        return Counter(s) == Counter(t)

word-break/njngwn.py

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+from functools import cache
+
+
+class Solution:
+    def wordBreak(self, s: str, wordDict: List[str]) -> bool:
+        @cache
+        def check(cur):
+            if cur == len(s):
+                return True
+            for word in wordDict:
+                if s[cur: cur + len(word)] == word:
+                    if check(cur + len(word)):
+                        return True
+            return False
+
+        return check(0)