946. Validate Stack Sequences (Python)

Related Topic

Stack.

Description

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.

Sample I/O

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.

Note

1. 0 <= pushed.length == popped.length <= 1000
2. 0 <= pushed[i], popped[i] < 1000
3. pushed is a permutation of popped.
4. pushed and popped have distinct values.

Methodology

We need a stack to store temperoray value. Iterate the pushed list and append the value into stack if the top value of stack is equal to the popped[index] value then pop the top from stack and update the index. After iteration, if the stack is empty we know all values are operated and return true

    def validateStackSequences(self, pushed: List[int], popped: List[int]) -> bool:
        stack = []
        index = 0
        for x in pushed:
            stack.append(x)
            while stack and stack[-1] == popped[index]:
                stack.pop()
                index += 1
        return not stack

BigO

We iterate the list once so the total cost will be O(n)