BrowserHistory.swift

class BrowserHistory {
    
    var history: [String]!
    var current: Int!
    var bound: Int!
    
    init(_ homepage: String) {
        history = [homepage]
        current = 0
        bound = 0
    }
    
    func visit(_ url: String) {
        current = current + 1
        if current == history.count {
            history.append(url)
        }else {
            history[current] = url
        }
        bound = current
    }
    
    func back(_ steps: Int) -> String {
        current = max(current - steps, 0)
        return history[current]
    }
    
    func forward(_ steps: Int) -> String {
        current = min(steps + current, bound)
        return history[current]
    }
}

/**
 * Your BrowserHistory object will be instantiated and called as such:
 * let obj = BrowserHistory(homepage)
 * obj.visit(url)
 * let ret_2: String = obj.back(steps)
 * let ret_3: String = obj.forward(steps)
 */