diff --git a/DecodeWays.java b/DecodeWays.java
new file mode 100644
index 00000000..c8b04a1e
--- /dev/null
+++ b/DecodeWays.java
@@ -0,0 +1,56 @@
+package main;
+
+//A message containing letters from A-Z is being encoded to numbers using the following mapping:
+
+//'A' -> 1
+//'B' -> 2
+//...
+//'Z' -> 26
+//tự viết
+//Example 1
+//Input: 34
+//Output: 1
+//Example 2
+//Input: 12
+//Output: 2
+//Example 3
+//Input: 16 
+//Output: 2 
+//Example 4
+//Input:299 
+//Output: 1
+//Example 5
+//Input: 53 
+//Output: 1
+
+//Given an encoded message containing digits, determine the total number of ways to decode it.
+
+//For example,
+//Given encoded message "12", it could be decoded as "AB" (1 2) or "L" (12).
+
+//The number of ways decoding "12" is 2.
+
+public class DecodeWays {
+ public static int numDecodings(String s) {
+     int n = s.length();
+
+     if(n == 0) {
+         return 0;
+     }
+     
+     int[] dp = new int[n + 1];
+     dp[n] = 1;
+     dp[n - 1] = s.charAt(n - 1) != '0' ? 1 : 0;
+     
+     for(int i = n - 2; i >= 0; i--) {
+         if(s.charAt(i) == '0') {
+             continue;
+         } else {
+             dp[i] = (Integer.parseInt(s.substring(i, i + 2)) <= 26) ? dp[i + 1] + dp[i + 2] : dp[i + 1];
+         }
+     }
+     
+     return dp[0];
+ }
+}
+
diff --git a/MainTest.java b/MainTest.java
new file mode 100644
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--- /dev/null
+++ b/MainTest.java
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+package test;
+import static org.junit.Assert.assertEquals;
+
+import org.junit.Test;
+
+import main.Power;
+
+public class MainTest {
+
+	@Test
+	void testExample1() {
+
+		assertEquals(true, Power.isPowerOfTwo(1));
+
+	}
+@Test
+	void testExample4() {
+
+        assertEquals(true, Power.isPowerOfTwo(0));
+
+	}
+@Test
+	void testExample5() {
+
+		assertEquals(true, Power.isPowerOfTwo(100));
+
+	}
+
+}
diff --git a/Power.java b/Power.java
new file mode 100644
index 00000000..8b953177
--- /dev/null
+++ b/Power.java
@@ -0,0 +1,11 @@
+package main;
+public class Power {
+	public static boolean isPowerOfTwo(int n) {
+        long i = 1;
+        while(i < n) {
+            i <<= 1;
+        }
+        
+        return i == n;
+    }
+}
diff --git a/ReverseWord.java b/ReverseWord.java
new file mode 100644
index 00000000..ed4ef408
--- /dev/null
+++ b/ReverseWord.java
@@ -0,0 +1,31 @@
+package main;
+//Given an input string, reverse the string word by word.
+//For example,
+//Given s = "the sky is blue",
+//return "blue is sky the".
+//tự viết
+//Example 1
+//Input: String a = "Hello I am Nhi"
+//Output: String a1 = "Nhi am I Hello"
+//Example 2
+//Input: String b = "I Love You"
+//Output: String b1 = "You Love I"
+//Example 3
+//Input: String c = "Sorry"
+//Output: String c1 = "Sorry"
+//Example 4
+//Input: String d = "Thank You"
+//Output: String d1 = "You Thank"
+
+public class ReverseWord {
+  public static String reverseWords(String s) {
+      String[] words = s.trim().split("\\s+");
+      String result = "";
+      for(int i = words.length - 1; i > 0; i--) {
+          result += words[i] + " ";
+      }
+      
+      return result + words[0];
+  }
+}
+
diff --git a/TestDecodeFacebook.java b/TestDecodeFacebook.java
new file mode 100644
index 00000000..10126e70
--- /dev/null
+++ b/TestDecodeFacebook.java
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+package test;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+import main.DecodeWays;
+
+public class TestDecodeFacebook {
+	@Test
+	public void  testEx1() {
+		
+		assertEquals(1, DecodeWays.numDecodings("34"));
+
+	}
+    @Test
+	public void testEx2() {
+		
+		assertEquals(2, DecodeWays.numDecodings("12"));
+
+	}
+    @Test
+	public void testEx5() {
+		
+		assertEquals(1, DecodeWays.numDecodings("53"));
+
+	}
+}
diff --git a/TestReverseWord.java b/TestReverseWord.java
new file mode 100644
index 00000000..4bfb9d1e
--- /dev/null
+++ b/TestReverseWord.java
@@ -0,0 +1,30 @@
+package test;
+import static org.junit.Assert.assertEquals;
+
+import org.junit.Test;
+
+import main.ReverseWord;
+
+
+public class TestReverseWord {
+	@Test
+	public void Example1TestReverse() {
+		
+			System.out.println(ReverseWord.reverseWords("Hello I am Nhi"));
+			assertEquals("Nhi am I Hello", ReverseWord.reverseWords("Hello I am Nhi"));
+			
+		
+	}
+@Test
+	public void Example3TestReverse() {
+		System.out.println(ReverseWord.reverseWords("Sorry"));
+        assertEquals("Sorry", ReverseWord.reverseWords("Sorry"));
+
+	}
+@Test
+	public void Example4TestReverse() {
+		System.out.println(ReverseWord.reverseWords("Thank You"));
+		assertEquals("You Thank", ReverseWord.reverseWords("Thank You"));
+
+	}
+}