Added reverse bits

Author: FreeTymeKiyan

Easy/ReverseBits.java

@@ -0,0 +1,41 @@
+import java.util.*;
+
+/**
+ * Reverse bits of a given 32 bits <strong>unsigned</strong> integer.
+ * 
+ * Example: 
+ * input 43261596, represented in binary as 00000010100101000001111010011100
+ * return 964176192, represented in binary as 00111001011110000010100101000000
+ * 
+ * Follow up:
+ * If this function is <strong>called many times</strong>, how would you 
+ * optimize it?
+ * 
+ * Related problem: Reverse Integer
+ * 
+ * Tags: Bit Manipulation
+ */
+class ReverseBits {
+    public static void main(String[] args) {
+        ReverseBits r = new ReverseBits();
+        int a = 43261596;
+        System.out.println(r.reverseBits(a));
+    }
+    
+    /**
+     * O(1) Time, O(1) Space
+     * Move res 1 bit left, a
+     * Get first bit of n, b
+     * res = a ^ b
+     * Move n right 1 bit for next loop
+     * Unsigned shift means fill new bit at the left with 0 instead of 1
+     */
+    public int reverseBits(int n) {
+        int res = 0;
+        for (int i = 0; i < 32; i++) {
+            res = (res << 1) ^ (n & 1); // add first bit of n to last bit of res
+            n >>>= 1; // unsigned shift to right
+        }
+        return res;
+    }
+}