LongestSubstringWithKDistinctCharacters done

Author: rampatra

src/main/java/com/leetcode/maps/slidingwindow/LongestSubstringWithKDistinctCharacters.java

@@ -0,0 +1,80 @@
+package com.leetcode.maps.slidingwindow;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/**
+ * Level: Hard
+ * Link: https://leetcode.com/problems/longest-substring-with-at-most-k-distinct-characters/
+ * Description:
+ * Given a string, find the length of the longest substring T that contains at most k distinct characters.
+ * <p>
+ * Example 1:
+ * Input: s = "eceba", k = 2
+ * Output: 3
+ * Explanation: T is "ece" which its length is 3.
+ * <p>
+ * Example 2:
+ * Input: s = "aa", k = 1
+ * Output: 2
+ * Explanation: T is "aa" which its length is 2.
+ *
+ * @author rampatra
+ * @since 2019-08-09
+ */
+public class LongestSubstringWithKDistinctCharacters {
+
+    /**
+     * Time Complexity: O(n)
+     * Space Complexity: O(k), as we keep at most k characters in the hash table
+     *
+     * @param str
+     * @param k
+     * @return
+     */
+    public static int lengthOfLongestSubstringKDistinct(String str, int k) {
+        int length = 0;
+        Map<Character, Integer> letterCountInWindow = new HashMap<>();
+
+        int i = 0; // start of window
+        int j = i; // end of window
+
+        while (j < str.length()) {
+            char ch = str.charAt(j);
+
+            letterCountInWindow.putIfAbsent(ch, 0);
+            letterCountInWindow.put(ch, letterCountInWindow.get(ch) + 1);
+
+            // when number of distinct characters in the window exceeds k:
+            // - update length
+            // - remove the first character in the window or reduce its count if the window had more than one of this character
+            // - lastly, move the window forward
+            if (letterCountInWindow.keySet().size() > k) {
+                char firstChar = str.charAt(i);
+                int firstCharCount = letterCountInWindow.get(firstChar);
+                if (firstCharCount > 1) {
+                    letterCountInWindow.put(firstChar, firstCharCount - 1);
+                } else {
+                    letterCountInWindow.remove(firstChar);
+                }
+                length = Math.max(length, j - i);
+                i++;
+            }
+            j++;
+        }
+
+        return length == 0 ? j - i : length;
+    }
+
+    public static void main(String[] args) {
+        assertEquals(3, lengthOfLongestSubstringKDistinct("eceba", 2));
+        assertEquals(7, lengthOfLongestSubstringKDistinct("eceeeeeba", 2));
+        assertEquals(2, lengthOfLongestSubstringKDistinct("abcdef", 2));
+        assertEquals(1, lengthOfLongestSubstringKDistinct("a", 1));
+        assertEquals(2, lengthOfLongestSubstringKDistinct("aa", 1));
+        assertEquals(3, lengthOfLongestSubstringKDistinct("aaa", 1));
+        assertEquals(0, lengthOfLongestSubstringKDistinct("aa", 0));
+    }
+}