Add 'Longest Substring with At Most K Distinct Characters'

Author: nickolashkraus

README.md

@@ -42,6 +42,8 @@ A collection of LeetCode solutions
 
 [Longest Repeating Character Replacement](./src/longest_repeating_character_replacement.py)
 
+[Longest Substring with At Most K Distinct Characters](./src/longest_substring_with_at_most_k_distinct_characters.py)
+
 [Longest Substring with At Most Two Distinct Characters](./src/longest_substring_with_at_most_two_distinct_characters.py)
 
 [Longest Substring Without Repeating Characters](./src/test_longest_substring_without_repeating_characters.py)

src/longest_substring_with_at_most_k_distinct_characters.py

@@ -0,0 +1,35 @@
+"""
+340. Longest Substring with At Most K Distinct Characters
+
+https://leetcode.com/problems/longest-substring-with-at-most-k-distinct-characters
+
+NOTES
+  * Use a sliding window.
+
+A general solution to 'Longest Substring with At Most Two Distinct Characters'.
+Simply substitute 'k' for '2'. Easy bag.
+"""
+
+class Solution:
+    def lengthOfLongestSubstringKDistinct(self, s: str, k: int) -> int:
+        left, right, maximum_size = 0, 0, 0
+
+        # Maintain a mapping of character -> index of last occurrence.
+        d: dict[str, int] = {}
+
+        while right < len(s):
+            d[s[right]] = right
+
+            # If the map ever contains more than 'k' keys, then the substring
+            # contains more than 'k' distinct characters.
+            if len(d) > k:
+                # Move `left` to the character after the next index.
+                next_index = min(d.values())
+                left = next_index + 1
+                # Remove the character from the hash map.
+                del d[s[next_index]]
+
+            maximum_size = max(maximum_size, right - left + 1)
+            right += 1
+
+        return maximum_size

tests/test_longest_substring_with_at_most_k_distinct_characters.py

@@ -0,0 +1,21 @@
+"""
+340. Longest Substring with At Most K Distinct Characters
+
+https://leetcode.com/problems/longest-substring-with-at-most-k-distinct-characters
+"""
+
+from unittest import TestCase
+
+from src.longest_substring_with_at_most_k_distinct_characters import (
+    Solution,
+)
+
+
+class TestSolution(TestCase):
+    def test_1(self):
+        exp = 3
+        assert Solution().lengthOfLongestSubstringKDistinct("eceba", 2) == exp
+
+    def test_2(self):
+        exp = 2
+        assert Solution().lengthOfLongestSubstringKDistinct("aa", 1) == exp