test: add LeetCode 1684 with tests, complexity docs, and migration

README.md

@@ -44,11 +44,11 @@ Each problem follows this structure:
 
 ## Solutions
 
-📊 **Statistics**: 54 problems solved (migrated to new structure)
+📊 **Statistics**: 55 problems solved (migrated to new structure)
 
 | Difficulty | Count |
 |------------|-------|
-| Easy       | 38    |
+| Easy       | 39    |
 | Medium     | 15    |
 | Hard       | 1     |
 

docs/EASY.md

@@ -1,6 +1,6 @@
 # Easy Problems
 
-Total: 38 problems solved
+Total: 39 problems solved
 
 ## Solutions
 
@@ -32,6 +32,7 @@ Total: 38 problems solved
 | 1603 | [Design Parking System](https://leetcode.com/problems/design-parking-system/) | [DesignParkingSystem1603.java](../src/main/java/com/leetcode/easy/DesignParkingSystem1603.java) | [DesignParkingSystem1603Test.java](../src/test/java/com/leetcode/easy/DesignParkingSystem1603Test.java) |
 | 1662 | [Check If Two String Arrays are Equivalent](https://leetcode.com/problems/check-if-two-string-arrays-are-equivalent/) | [CheckIfTwoStringArraysAreEquivalent1662.java](../src/main/java/com/leetcode/easy/CheckIfTwoStringArraysAreEquivalent1662.java) | [CheckIfTwoStringArraysAreEquivalent1662Test.java](../src/test/java/com/leetcode/easy/CheckIfTwoStringArraysAreEquivalent1662Test.java) |
 | 1672 | [Richest Customer Wealth](https://leetcode.com/problems/richest-customer-wealth/) | [RichestCustomerWealth1672.java](../src/main/java/com/leetcode/easy/RichestCustomerWealth1672.java) | [RichestCustomerWealth1672Test.java](../src/test/java/com/leetcode/easy/RichestCustomerWealth1672Test.java) |
+| 1684 | [Count the Number of Consistent Strings](https://leetcode.com/problems/count-the-number-of-consistent-strings/) | [CountTheNumberOfConsistentStrings1684.java](../src/main/java/com/leetcode/easy/CountTheNumberOfConsistentStrings1684.java) | [CountTheNumberOfConsistentStrings1684Test.java](../src/test/java/com/leetcode/easy/CountTheNumberOfConsistentStrings1684Test.java) |
 | 1773 | [Count Items Matching a Rule](https://leetcode.com/problems/count-items-matching-a-rule/) | [CountItemsMatchingARule1773.java](../src/main/java/com/leetcode/easy/CountItemsMatchingARule1773.java) | [CountItemsMatchingARule1773Test.java](../src/test/java/com/leetcode/easy/CountItemsMatchingARule1773Test.java) |
 | 1816 | [Truncate Sentence](https://leetcode.com/problems/truncate-sentence/) | [TruncateSentence1816.java](../src/main/java/com/leetcode/easy/TruncateSentence1816.java) | [TruncateSentence1816Test.java](../src/test/java/com/leetcode/easy/TruncateSentence1816Test.java) |
 | 1941 | [Check if All Characters Have Equal Number of Occurrences](https://leetcode.com/problems/check-if-all-characters-have-equal-number-of-occurrences/) | [CheckIfAllCharactersHaveEqualNumberOfOccurrences1941.java](../src/main/java/com/leetcode/easy/CheckIfAllCharactersHaveEqualNumberOfOccurrences1941.java) | [CheckIfAllCharactersHaveEqualNumberOfOccurrences1941Test.java](../src/test/java/com/leetcode/easy/CheckIfAllCharactersHaveEqualNumberOfOccurrences1941Test.java) |

src/main/java/com/leetcode/easy/CountTheNumberOfConsistentStrings1684.java

@@ -0,0 +1,47 @@
+// Tags: Array, String, Hash Set
+package com.leetcode.easy;
+
+import java.util.HashSet;
+import java.util.Set;
+
+public class CountTheNumberOfConsistentStrings1684 {
+
+    /**
+     * Counts the number of consistent strings in the given array.
+     * A string is consistent if all its characters appear in the allowed string.
+     *
+     * <p>Algorithm:
+     * <ol>
+     *   <li>Convert allowed string to a HashSet for O(1) character lookup</li>
+     *   <li>For each word, check if all characters exist in the allowed set</li>
+     *   <li>Count words that pass the consistency check</li>
+     * </ol>
+     *
+     * <p>Time Complexity: O(m + n×k) where m = allowed.length(), n = words.length, k = average word length
+     * <br>Space Complexity: O(m) for storing the allowed characters in HashSet
+     *
+     * @param allowed String containing all allowed characters
+     * @param words   Array of strings to check for consistency
+     * @return The number of consistent strings
+     */
+    public int countConsistentStrings(String allowed, String[] words) {
+        int res = 0;
+        Set<Character> allowedCharacters = new HashSet<>();
+        for (int i = 0; i < allowed.length(); i++) {
+            allowedCharacters.add(allowed.charAt(i));
+        }
+        for (String word : words) {
+            boolean isConsistent = true;
+            for (int i = 0; i < word.length(); i++) {
+                if (!allowedCharacters.contains(word.charAt(i))) {
+                    isConsistent = false;
+                    break;
+                }
+            }
+            if (isConsistent) {
+                res++;
+            }
+        }
+        return res;
+    }
+}

src/test/java/com/leetcode/easy/CountTheNumberOfConsistentStrings1684Test.java

@@ -0,0 +1,46 @@
+package com.leetcode.easy;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class CountTheNumberOfConsistentStrings1684Test {
+
+    @Test
+    void testExample1() {
+        CountTheNumberOfConsistentStrings1684 solution = new CountTheNumberOfConsistentStrings1684();
+        String allowed = "ab";
+        String[] words = {"ad", "bd", "aaab", "baa", "badab"};
+        int expected = 2;
+
+        int result = solution.countConsistentStrings(allowed, words);
+
+        assertEquals(expected, result,
+            "Strings \"aaab\" and \"baa\" are consistent since they only contain characters 'a' and 'b'");
+    }
+
+    @Test
+    void testExample2() {
+        CountTheNumberOfConsistentStrings1684 solution = new CountTheNumberOfConsistentStrings1684();
+        String allowed = "abc";
+        String[] words = {"a", "b", "c", "ab", "ac", "bc", "abc"};
+        int expected = 7;
+
+        int result = solution.countConsistentStrings(allowed, words);
+
+        assertEquals(expected, result,
+            "All strings should be consistent");
+    }
+
+    @Test
+    void testExample3() {
+        CountTheNumberOfConsistentStrings1684 solution = new CountTheNumberOfConsistentStrings1684();
+        String allowed = "cad";
+        String[] words = {"cc", "acd", "b", "ba", "bac", "bad", "ac", "d"};
+        int expected = 4;
+
+        int result = solution.countConsistentStrings(allowed, words);
+
+        assertEquals(expected, result,
+            "Strings \"cc\", \"acd\", \"ac\", and \"d\" are consistent");
+    }
+}