LeetCode in Kotlin

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.

Solution

import java.util.Deque
import java.util.LinkedList

class Solution {
    fun validateStackSequences(pushed: IntArray, popped: IntArray): Boolean {
        val stack: Deque<Int> = LinkedList()
        var i = 0
        var j = 0
        val len = pushed.size
        while (i < len) {
            if (pushed[i] == popped[j]) {
                i++
                j++
            } else if (stack.isNotEmpty() && stack.peek() == popped[j]) {
                stack.pop()
                j++
            } else {
                stack.push(pushed[i++])
            }
        }
        while (j < len) {
            if (stack.isNotEmpty() && stack.peek() != popped[j++]) {
                return false
            } else {
                stack.pop()
            }
        }
        return true
    }
}

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