946. Validate Stack Sequences(Medium)

Given two integer arrays pushed and popped each with distinct values, return true if this could have been the result of a sequence of push and pop operations on an initially empty stack, or false otherwise.

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:

1 <= pushed.length <= 1000

0 <= pushed[i] <= 1000

All the elements of pushed are unique.

popped.length == pushed.length

popped is a permutation of pushed.

class Solution:

def validateStackSequences(self, pushed: List[int], popped: List[int]) -> bool:

stack = []

i = 0 # popped's index

for x in pushed:

stack.append(x)

while stack and stack[-1] == popped[i]:

stack.pop()

i += 1

return not stack