Add solution for the "Jewels and Stones" problem with unit tests.

src/main/java/com/leetcode/easy/JewelsAndStones771.java

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+// Tags: String, Hash Set
+package com.leetcode.easy;
+
+import java.util.HashSet;
+import java.util.Set;
+
+public class JewelsAndStones771 {
+    /*
+    Time complexity: O(J + S)
+    Breakdown:
+    -  First loop: Iterates through the jewels string of length J, adding each character to the HashSet
+       -  Each add() operation on HashSet takes O(1) average time
+       -  Total: O(J)
+    -  Second loop: Iterates through the stones string of length S, checking if each character exists in the HashSet
+       -  Each contains() operation on HashSet takes O(1) average time
+       -  Total: O(S)
+    -  Overall: O(J) + O(S) = O(J + S)
+    Where J is the length of the jewels string and S is the length of the stones string.
+
+    Space complexity: O(J)
+    Breakdown:
+    -  HashSet storage: The jewelsTypes HashSet stores at most J unique characters from the jewels string
+        -  In the worst case, all characters in jewels are unique, requiring O(J) space
+    -  Other variables: res and loop variables use O(1) constant space
+    -  Overall: O(J)
+     */
+    public int numJewelsInStones(String jewels, String stones) {
+        Set<Character> jewelsTypes = new HashSet<>();
+        for (int i = 0; i < jewels.length(); i++) {
+            jewelsTypes.add(jewels.charAt(i));
+        }
+        int res = 0;
+        for (int i = 0; i < stones.length(); i++) {
+            if (jewelsTypes.contains(stones.charAt(i))) {
+                res++;
+            }
+        }
+        return res;
+    }
+}

src/test/java/com/leetcode/easy/JewelsAndStones771Test.java

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+package com.leetcode.easy;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class JewelsAndStones771Test {
+    private JewelsAndStones771 solution;
+
+    @BeforeEach
+    void setUp() {
+        solution = new JewelsAndStones771();
+    }
+
+    @Test
+    void testNumJewelsInStones_Example1() {
+        String jewels = "aA";
+        String stones = "aAAbbbb";
+
+        int result = solution.numJewelsInStones(jewels, stones);
+
+        assertEquals(3, result);
+    }
+
+    @Test
+    void testNumJewelsInStones_Example2() {
+        String jewels = "z";
+        String stones = "ZZ";
+
+        int result = solution.numJewelsInStones(jewels, stones);
+
+        assertEquals(0, result);
+    }
+}