[njngwn] WEEK 08 Solutions

longest-repeating-character-replacement/njngwn.py

@@ -0,0 +1,21 @@
+class Solution:
+    # Time Complexity: O(n), n: len(s)
+    # Space Complexity: O(1)
+    def characterReplacement(self, s: str, k: int) -> int:
+        count = [0] * 26
+        max_len, max_cnt = 0, 0
+        left = 0
+
+        for right in range(len(s)):  # window expands
+            ch = ord(s[right]) - ord('A')
+            count[ch] += 1
+            max_cnt = max(max_cnt, count[ch])
+
+            # len(substring) - len(most frequent character) > k => window needs to schrink
+            if (right - left + 1) - max_cnt > k:
+                count[ord(s[left]) - ord('A')] -= 1
+                left += 1
+
+            max_len = max(max_len, right - left + 1)
+
+        return max_len

palindromic-substrings/njngwn.py

@@ -0,0 +1,22 @@
+class Solution:
+    # Time Complexity: O(n^2), n: len(s)
+    # Space Complexity: O(1)
+    def countSubstrings(self, s: str) -> int:
+        cnt = 0
+
+        for i in range(len(s)):
+            # odd number
+            start, end = i, i
+            while start >= 0 and end < len(s) and s[start] == s[end]:
+                start -= 1
+                end += 1
+                cnt += 1
+
+            # even number
+            start, end = i, i + 1
+            while start >= 0 and end < len(s) and s[start] == s[end]:
+                start -= 1
+                end += 1
+                cnt += 1
+
+        return cnt

reverse-bits/njngwn.py

@@ -0,0 +1,15 @@
+class Solution:
+    # Time Complexity: O(1)
+    # Space Complexity: O(1)
+    def reverseBits(self, n: int) -> int:
+        stack = []
+        for i in range(32):
+            stack.append(n % 2)
+            n //= 2
+
+        res, multiples = 0, 1
+        while stack:
+            res += (stack.pop() * multiples)
+            multiples *= 2
+
+        return res