백준 - 스택 수열

let n = Int(readLine()!)!
var current = 1
var stack = [Int]()
var answer = ""

for _ in 0..<n {
    let input = Int(readLine()!)!
    
    if input >= current {
        while input >= current {
            stack.append(current)
            answer += "+\n"
            current += 1
        }
        stack.removeLast()
        answer += "-\n"
    } else {
        if stack.last! == input {
            stack.removeLast()
            answer += "-\n"
        } else {
            answer = "NO"
            break
        }
    }
}

print(answer)