## DEV Community

Abhishek Chaudhary

Posted on

# Validate Stack Sequences

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`.

SOLUTION:

``````class Solution:
def validateStackSequences(self, pushed: List[int], popped: List[int]) -> bool:
n = len(popped)
i = 0
j = 0
stack = []
while j < n:
while len(stack) == 0 or stack[-1] != popped[j]:
if i >= n:
return False
stack.append(pushed[i])
i += 1
stack.pop()
j += 1
return True
``````