0946.Validate_Stack_Sequences.py

"""
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:
        st = []
        i = 0
        for nums in pushed:
            st.append(nums)
            while len(st) > 0 and st[-1] == popped[i]:
                st.pop()
                i += 1
        return len(st) == 0