How do you solve Validate Stack Sequences on LeetCode?

Validate Stack Sequences (LeetCode #946) can be solved using Brute Force or Optimal Solution. The optimal approach is Optimal Solution with O(n) time complexity and O(n) space complexity. Key insight: The order of popping must always respect the last-in-first-out (LIFO) nature of the stack.

What is the brute force solution for Validate Stack Sequences?

The brute force approach for Validate Stack Sequences is Brute Force, with O(n²) time complexity and O(1) space complexity. We can simulate the stack operations by pushing elements from the 'pushed' array and checking if we can pop them in the order specified by the 'popped' array. This approach is straightforward but inefficient. The optimal Optimal Solution approach improves this to O(n).

What algorithm or data structure is used to solve Validate Stack Sequences?

Validate Stack Sequences uses the following concepts: Array, Stack, Simulation. The recommended patterns to study are: Stack, Simulation.

What are the interview tips for Validate Stack Sequences?

Always clarify the problem constraints and requirements before diving into coding. Talk through your thought process and explain your approach as you code. Consider edge cases, such as when the arrays are empty or contain only one element.

What are common mistakes when solving Validate Stack Sequences?

Not considering the order of operations correctly. Assuming all elements can be popped in any order without following stack rules.

#946

Validate Stack Sequences

Medium

ArrayStackSimulationStackSimulation

LeetCode ↗

Approaches

Brute ForceOptimal

Complexity Comparison

	Brute Force	Optimal Solution★
Time	O(n²)	O(n)
Space	O(1)	O(n)

💡

Intuition

Time O(n)Space O(n)

We can use a stack to simulate the push and pop operations efficiently. By maintaining a pointer for the popped array, we can determine if the sequence is valid in linear time.

⚙️

Algorithm

4 steps

1Step 1: Initialize an empty stack and a pointer for the popped array.
2Step 2: Iterate through each element in the pushed array, pushing elements onto the stack.
3Step 3: After each push, check if the top of the stack matches the current element in the popped array. If it does, pop it and move the pointer in the popped array.
4Step 4: At the end, check if all elements have been popped.

solution.py9 lines

1def validateStackSequences(pushed, popped):
2    stack = []
3    j = 0
4    for x in pushed:
5        stack.append(x)
6        while stack and stack[-1] == popped[j]:
7            stack.pop()
8            j += 1
9    return j == len(popped)

ℹ

Complexity note: The time complexity is O(n) because we traverse each element in the pushed array once, and the space complexity is O(n) due to the stack storing elements.

1The order of popping must always respect the last-in-first-out (LIFO) nature of the stack.
2The pushed and popped arrays must contain the same elements in different orders.

Solutions and explanations are original Tejav content. Problem titles © LeetCode — use the LeetCode button above for the full problem statement.