Add FirstUniqueCharacterInString task solution

Author: andrei-punko

src/main/java/by/andd3dfx/string/FirstUniqueCharacterInString.java

@@ -0,0 +1,43 @@
+package by.andd3dfx.string;
+
+import java.util.LinkedHashMap;
+
+/**
+ * <pre>
+ * <a href="https://leetcode.com/problems/first-unique-character-in-a-string/description/">Task description</a>
+ *
+ * Given a string s, find the first non-repeating character in it and return its index. If it does not exist, return -1.
+ *
+ * Example 1:
+ * Input: s = "leetcode"
+ * Output: 0
+ * Explanation: The character 'l' at index 0 is the first character that does not occur at any other index.
+ *
+ * Example 2:
+ * Input: s = "loveleetcode"
+ * Output: 2
+ *
+ * Example 3:
+ * Input: s = "aabb"
+ * Output: -1
+ * </pre>
+ */
+public class FirstUniqueCharacterInString {
+
+    public static int firstUniqChar(String s) {
+        LinkedHashMap<Character, Integer> map = new LinkedHashMap<>();
+        for (var ch : s.toCharArray()) {
+            if (map.containsKey(ch)) {
+                map.put(ch, map.get(ch) + 1);
+            } else {
+                map.put(ch, 1);
+            }
+        }
+        for (var entry : map.entrySet()) {
+            if (entry.getValue() == 1) {
+                return s.indexOf(entry.getKey());
+            }
+        }
+        return -1;
+    }
+}

src/test/java/by/andd3dfx/string/FirstUniqueCharacterInStringTest.java

@@ -0,0 +1,18 @@
+package by.andd3dfx.string;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.Test;
+
+public class FirstUniqueCharacterInStringTest {
+
+    @Test
+    public void firstUniqChar() {
+        assertThat(FirstUniqueCharacterInString.firstUniqChar("leetcode"))
+            .isEqualTo(0);
+        assertThat(FirstUniqueCharacterInString.firstUniqChar("loveleetcode"))
+            .isEqualTo(2);
+        assertThat(FirstUniqueCharacterInString.firstUniqChar("aabb"))
+            .isEqualTo(-1);
+    }
+}