diff --git a/src/main/java/com/fishercoder/solutions/_150.java b/src/main/java/com/fishercoder/solutions/_150.java
index c8e7abfbd9..c03cc3bd49 100644
--- a/src/main/java/com/fishercoder/solutions/_150.java
+++ b/src/main/java/com/fishercoder/solutions/_150.java
@@ -62,4 +62,30 @@ public int evalRPN(String[] tokens) {
         return Integer.parseInt(stack.pop());
     }
 
+    //using one stack only.
+    public int evalRPN1(String[] tokens) {
+        Stack<Integer> stack =  new Stack<Integer>();
+        for (int i = 0; i < tokens.length; i++) {
+            if (tokens[i].equals("+")) {
+                stack.push(stack.pop() + stack.pop());
+            }
+            else if (tokens[i].equals("-")) {
+                stack.push(-stack.pop() + stack.pop());
+            }
+            else if (tokens[i].equals("*") ) {
+                stack.push(stack.pop() * stack.pop());
+
+            }
+            else if (tokens[i].equals("/")) {
+                int a = stack.pop();
+                int b = stack.pop();
+                stack.push(b / a);
+
+            } else {
+                stack.push(Integer.parseInt (tokens[i]));
+            }
+        }
+        return stack.pop();
+    }
+
 }
diff --git a/src/main/java/com/fishercoder/solutions/_344.java b/src/main/java/com/fishercoder/solutions/_344.java
index 91a6549653..102007bdc8 100644
--- a/src/main/java/com/fishercoder/solutions/_344.java
+++ b/src/main/java/com/fishercoder/solutions/_344.java
@@ -12,20 +12,15 @@ public String reverseString_cheating(String s) {
         return new StringBuilder(s).reverse().toString();
     }
     
-    public String reverseString(String s) {
+    public String reverseString(String s){
         int i = 0, j = s.length()-1;
         char[] chars = s.toCharArray();
         while(i < j){
             char temp = chars[i];
-            chars[i] = chars[j];
-            chars[j] = temp;
-            i++;
-            j--;
-        }
-        StringBuilder sb = new StringBuilder();
-        for(char c : chars){
-            sb.append(c);
+            chars[i++] = chars[j];
+            chars[j--] = temp;
+
         }
-        return sb.toString();
+        return new String(chars);
     }
 }