add max stack problem

Author: sagivo

README.md

@@ -42,6 +42,7 @@ This is my way of saying - change your interview style. There are lots of smart
 | [count and say](https://leetcode.com/problems/count-and-say/) | [click](https://github.com/sagivo/algorithms/blob/master/src/count_and_say.rb) |
 | [Telephone number to words](http://www.mobilefish.com/services/phonenumber_words/phonenumber_words.php) | [click](https://github.com/sagivo/algorithms/blob/master/src/phone.rb) |
 | [Maximum contiguous subarray](https://leetcode.com/problems/maximum-subarray) | [click](https://github.com/sagivo/algorithms/blob/master/src/max_subarray.rb) |
+| [Max Stack](https://leetcode.com/problems/max-stack/submissions/) | [click](https://github.com/sagivo/algorithms/blob/master/src/max_stack.rb) |
 | [Find the smallest biggest number that has the same digits](http://stackoverflow.com/questions/9368205/given-a-number-find-the-next-higher-number-which-has-the-exact-same-set-of-digi) | [click](https://github.com/sagivo/algorithms/blob/master/src/bigger_num_with_same_digits.rb) |
 | [Find the minimum insertions needed to make a word palindrome](http://www.geeksforgeeks.org/dynamic-programming-set-28-minimum-insertions-to-form-a-palindrome/) | [click](https://github.com/sagivo/algorithms/blob/master/src/min_insertions_for_palindrome.rb) |
 | [String matching - Knuth Morris Pratt algorithm KMP](http://en.wikipedia.org/wiki/Knuth%E2%80%93Morris%E2%80%93Pratt_algorithm) | [click](https://github.com/sagivo/algorithms/blob/master/src/kmp.rb) |

src/max_stack.rb

@@ -0,0 +1,46 @@
+class MaxStack
+ def initialize()
+ @q = []
+ @q2 = []
+ end
+
+ def push(x)
+ max = @q.any? ? [x, @q.last[1]].max : x
+ @q << [x, max]
+ x
+ end
+
+ def pop()
+ return nil unless @q.any?
+
+ ret = @q.pop[0]
+ ret
+ end
+
+ def top()
+ return nil unless @q.any?
+ @q.last[0]
+ end
+
+ def peek_max()
+ return nil unless @q.any?
+ @q.last[1]
+ end
+
+
+ def pop_max()
+ return nil unless @q.any?
+ max = @q.last[1]
+ q2 = []
+
+ while top() != max
+ q2 << pop()
+ end
+ pop()
+
+ while q2.any?
+ push(q2.pop)
+ end
+ max
+ end
+end