chore(Java): add reverse string (#591)

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)
+- [Reverse String](strings/reverse-string.java)
 - [First Non Repeating Character](strings/first-non-repeating-char.java)
 
 ## Trees

algorithms/Java/strings/reverse-string.java

@@ -0,0 +1,36 @@
+/*
+Write a function that reverses a string. The input string is given as an array of characters s.
+sample input:
+Hello
+
+sample output:
+olleH
+
+Time Complexity: O(n)
+*/
+
+package stringProblems;
+
+import java.util.*;
+
+public class ReverseString {
+	public static String revString(String str) {
+	    String reversedString = "";
+        
+	    for (int i = 0; i < str.length(); i++) {
+	    	reversedString = str.charAt(i) + reversedString;
+	    }
+	    return reversedString;
+}
+	
+	public static void main(String[] args) {
+		Scanner s = new Scanner(System.in);
+		
+		String str = s.nextLine();
+		
+		String x = revString(str);
+		System.out.print(x);
+		
+	}
+
+}