text: # Source : https://leetcode.com/problems/validate-stack-sequences/
  # Author : William Heryanto
  # Date   : 2021-02-26
  
  ###############################################################################
  #
  # 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.
  #
  ###############################################################################
  
  class Solution:
      def validateStackSequences(self, pushed: List[int], popped: List[int]) -> bool:
          if not pushed or not popped:
              return True
  
          l, r, n = 1, 0, len(pushed) - 1
          stack = [pushed[0]]
  
          while l <= n or r <= n:
              try:
                  if stack and stack[-1] == popped[r]:
                      stack.pop()
                      r += 1
                  else:
                      stack.append(pushed[l])
                      l += 1
              except IndexError:
                  return False
  
          return True
  