validateStackSequences.swift

/*
 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.

 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.
  */
func validateStackSequences(_ pushed: [Int], _ popped: [Int]) -> Bool {
    var stack = [Int]()
    var j = 0
    for i in pushed {
        stack.append(i)
        while !stack.isEmpty && j < pushed.count && stack.last! == popped[j] {
            stack.removeLast()
            j = j + 1
        }
    }
    return j == pushed.count
}