From 7d4ee3c943d7dae974a06735c586884ea086c56c Mon Sep 17 00:00:00 2001
From: Piyush Kumar <220101130066@cutm.ac.in>
Date: Wed, 8 Oct 2025 13:49:50 +0530
Subject: [PATCH] Add Java solution for First Non-Repeated Character with
 explanation and examples

---
 java/strings/FirstNonRepeatedChar.java | 66 ++++++++++++++++++++++++++
 1 file changed, 66 insertions(+)
 create mode 100644 java/strings/FirstNonRepeatedChar.java

diff --git a/java/strings/FirstNonRepeatedChar.java b/java/strings/FirstNonRepeatedChar.java
new file mode 100644
index 00000000..ce4a7dfa
--- /dev/null
+++ b/java/strings/FirstNonRepeatedChar.java
@@ -0,0 +1,66 @@
+import java.util.LinkedHashMap;
+import java.util.Map;
+
+/**
+ * Problem: Find the First Non-Repeated Character in a String
+ *
+ * Given a string, the task is to find the first character that does not repeat.
+ *
+ * Examples:
+ * Input: "swiss"
+ * Output: "First non-repeated character: w"
+ *
+ * Input: "hello"
+ * Output: "First non-repeated character: h"
+ *
+ * Approach:
+ * 1. Use a LinkedHashMap to store the frequency of each character.
+ *    - Key: Character
+ *    - Value: Count of occurrences
+ * 2. LinkedHashMap maintains the insertion order, unlike HashMap.
+ *    This ensures we check characters in the order they appear.
+ * 3. Iterate through the string and populate the map with counts.
+ * 4. Iterate through the map entries:
+ *    - The first character with a count of 1 is the answer.
+ *
+ * Time Complexity: O(n) 
+ * Space Complexity: O(n)
+ */
+public class FirstNonRepeatedChar {
+
+    public static void main(String[] args) {
+
+        // Test cases
+        String[] testStrings = {"swiss", "hello", "aabbcc", "programming"};
+
+        for (String s : testStrings) {
+            System.out.println("Input: " + s);
+            System.out.println("Output: " + firstNonRepeated(s));
+            System.out.println("-------------------------");
+        }
+    }
+
+    /**
+     * Finds the first non-repeated character in the string
+     * @param str input string
+     * @return first non-repeated character as a String, or message if none found
+     */
+    public static String firstNonRepeated(String str) {
+        LinkedHashMap<Character, Integer> charCountMap = new LinkedHashMap<>();
+
+        // Count frequency of each character
+        for (int i = 0; i < str.length(); i++) {
+            char ch = str.charAt(i);
+            charCountMap.put(ch, charCountMap.getOrDefault(ch, 0) + 1);
+        }
+
+        // Find the first character with count 1
+        for (Map.Entry<Character, Integer> entry : charCountMap.entrySet()) {
+            if (entry.getValue() == 1) {
+                return "First non-repeated character: " + entry.getKey();
+            }
+        }
+
+        return "No non-repeated character found";
+    }
+}