validateStackSequences.py

'''946. Validate Stack Sequences

Given two sequences pushed and popped with distinct values, return true if and only if this could have been the result of a sequence of push and pop 
operations on an initially empty stack.

Example 1:

Input: pushed = [1,2,3,4,5], popped = [4,5,3,2,1]
Output: true
Explanation: We might do the following sequence:
push(1), push(2), push(3), push(4), pop() -> 4,
push(5), pop() -> 5, pop() -> 3, pop() -> 2, pop() -> 1

Example 2:

Input: pushed = [1,2,3,4,5], popped = [4,3,5,1,2]
Output: false
Explanation: 1 cannot be popped before 2.
 

Constraints:

0 <= pushed.length == popped.length <= 1000
0 <= pushed[i], popped[i] < 1000
pushed is a permutation of popped.
pushed and popped have distinct values.

'''
def validateStackSequences(self, pushed: List[int], popped: List[int]) -> bool:
  stack = []
  size = len(popped)
  j = 0
  for ele in pushed:
      stack.append(ele)
      #print(stack)
      while popped and stack and popped[j]==stack[-1]:
          j+=1
          stack.pop()
      #print('after popping',stack)
  if j==size:
      return True
  else:
      return False