Check Given Push-Pop Sequences in Python

Suppose we have a list of numbers called pushes, and another list of numbers called pops, we have to check whether this is a valid sequence of stack push and pop actions or not.

So, if the input is like pushes = [1, 2, 5, 7, 9] pops = [2, 1, 9, 7, 5], then the output will be True, as we can push [1, 2] first then pop them both. Then push [5, 7, 9] and pop them all.

To solve this, we will follow these steps −

• s := a new stack
• i := 0
• for each ele in pushes, do
  • push ele into s
  • while size of s > 0 and pops[i] is same top element of s, do
    • delete top element from s
    • i := i + 1
• return true when size of s is same as 0, otherwise false

Let us see the following implementation to get better understanding −

Example 

Live Demo

class Solution:
   def solve(self, pushes, pops):
      s = []
      i = 0

      for ele in pushes:
         s.append(ele)

         while len(s) > 0 and pops[i] == s[-1]:
            s.pop()
            i += 1

      return len(s) == 0

ob = Solution()
pushes = [1, 2, 5, 7, 9]
pops = [2, 1, 9, 7, 5]
print(ob.solve(pushes, pops))

Input

[1, 2, 5, 7, 9], [2, 1, 9, 7, 5]

Output

True