chore(Java): add first non repeating character (#596)

algorithms/Java/README.md

@@ -72,6 +72,7 @@
 - [Anagram](strings/anagram.java)
 - [Longest Common Substring](strings/Longest_common_substring.java)
 - [Boyer Moore Search](strings/Boyer_Moore.java)
+- [First Non Repeating Character](strings/first-non-repeating-char.java)
 
 ## Trees
 - [Pre in Post Traversal](trees/pre-in-post-traversal.java)

algorithms/Java/strings/first-non-repeating-char.java

@@ -0,0 +1,61 @@
+/*
+Output the first character which occurred once in the string.
+The input string can consist of any characters, like %,? or ü.
+There are no constraints. For examples, see the main function.
+Time Complexity: O(n)
+Explanation of the algorithm:
+             Iterating through the string twice:
+             First time, the program counts occurrences into a HashMap.
+             Second time, the program looks for the first character
+             in the string with the HashMap value of 1. */
+
+import java.util.*;
+
+public class FirstNonRepeatingChar {
+
+    // Contains examples of usage of the algorithm
+    public static void main(String[] args) {
+        String example1 = "abcbadabfaa334";
+        System.out.println(firstNonRepeatingChar(example1));
+        // Output: c
+
+        String example2 = "aab!b?cc!dü";
+        System.out.println(firstNonRepeatingChar(example2));
+        // Output: ?
+
+        String example3 = "aa!b?cc!ddb?";
+        System.out.println(firstNonRepeatingChar(example3));
+        // Output: null
+
+        String example4 = "üaa55vv22??1.";
+        System.out.println(firstNonRepeatingChar(example4));
+        // Output: ü
+
+    }
+
+    /*
+    Return the first character in string a that only occurs in string a once.
+    If every character in the string occurs more than once, return null.
+     */
+    public static Character firstNonRepeatingChar(String a) {
+        // map characters to the number of occurrences in the string
+        HashMap<Character, Integer> count = new HashMap<>();
+
+        // To iterate through the string, turn it to an array of characters
+        char[] charArray = a.toCharArray();
+
+        // Count occurrences of each character
+        for (Character i : charArray) {
+            Integer value = count.putIfAbsent(i, 0);
+            if (value == null) count.put(i, 1);
+            else count.put(i, value + 1);
+        }
+        // Search for the first matching character
+        for (Character k : charArray) {
+            if (count.get(k) == 1) {
+                return k;
+            }
+        }
+        return null;
+    }
+}