diff --git a/README.md b/README.md
index 268dd4e..2bd45f6 100644
--- a/README.md
+++ b/README.md
@@ -56,6 +56,8 @@ Collection of interesting questions and solutions that involve data structures,
 10. [Find min value of two unsorted lists](https://github.com/harishvc/challenges/blob/master/min-value-two-unsorted-lists.py) :bulb: :thumbsup:
 11. [Given a string where each character can be [0-9] or [+-*] find the result](https://github.com/harishvc/challenges/blob/master/stack-calculator.py) :thumbsup:
 12. [Find top 3 and bottom 2 values from the list in O(n)](https://github.com/harishvc/challenges/blob/master/find-top-3-bottom-3-values-in-a-list.py) :thumbsup:  
+13. Number of perfect squares between two given numbers  
+
 
 ### Brain Teasers (Moderate)
 1. [Check if a given number is prime](https://github.com/harishvc/challenges/blob/master/prime.py)
@@ -69,6 +71,7 @@ Collection of interesting questions and solutions that involve data structures,
 9. Matrix    
    9.1 [Print the matrix from outside to inside (Spiral)](https://github.com/harishvc/challenges/blob/master/matrix-spiral-print.py) :boom:   
    9.2 [Given a two dimentional matrix where rows and columns are sorted in increasing order. Design an efficient algorithm that decides whether a number X appears in A](https://github.com/harishvc/challenges/blob/master/two-dimensional-array-number-exist.py) :thumbsup:     
+10. Compute largest rectangle area under a histogram  
 
 ### Lists & Dictionaries
 1. [Randomize list elements](https://github.com/harishvc/challenges/blob/master/randomize-array-elements.py)
@@ -92,7 +95,8 @@ Collection of interesting questions and solutions that involve data structures,
    9.5 [Given two arrays of integers, find a pair of values (one from each array) you can swap so that both arrays sum to the same value] (https://github.com/harishvc/challenges/blob/master/lists-swap-same-sum.py)  [TODO]  
    9.6 [Find three numbers when multipled provide max value from the given input](https://github.com/harishvc/challenges/blob/master/find-three-numbers-when-multipled-provide-max-value.py)  
    9.7 Find all possible quadruplets in the list that add up to the given sum   
-   9.8 Find a continuous subarray of sum zero  
+   9.8 Find a continuous subarray of sum zero    
+   9.9 [Print alternate positive and negative values while retaining order](https://github.com/harishvc/challenges/blob/master/list-alternate-positive-negative.py)  :boom:    
 10. Rotated list   
    10.1 [Rotate a list to the right or left by n places](https://github.com/harishvc/challenges/blob/master/rotate-array.py)    
    10.2 [Find minimum in a rotated sorted list](https://github.com/harishvc/challenges/blob/master/rotate-list-minimum.py) :thumbsup:  
diff --git a/list-alternate-positive-negative.py b/list-alternate-positive-negative.py
new file mode 100644
index 0000000..a9b1fc2
--- /dev/null
+++ b/list-alternate-positive-negative.py
@@ -0,0 +1,46 @@
+
+#Given a list containing postive and negative values print values
+#alternatively (postive and negative or vise versa) while retaining 
+#order and contant space
+
+def alternateValues(a):
+	size = len(a)
+	for current in range(0,size-1,2): #increment by 2
+		i  = current+1
+		if a[current] > 0:
+			#find next negative
+			while i < size and a[i] > 0:
+				i += 1
+		else:
+			#find next positive
+			while i < size and a[i] < 0:
+				i += 1
+		#next two values are alternating
+		if (i < size):
+			a[current+1],a[i] = a[i],a[current+1]
+
+a = [-2,3,-4,9,-1,-7,1,5]
+print("input >>> ", a)
+alternateValues(a)
+print("result >>> ", a)
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