let mList = "[{\"content\":\"hello\"},{\"content\":\"hi\"},{\"content\":\"how are you?\"}]"
let JSONData = mList.dataUsingEncoding(NSUTF8StringEncoding)!
var json: Array<AnyObject>!
do {
    json = try NSJSONSerialization.JSONObjectWithData(JSONData, options: NSJSONReadingOptions()) as? Array
} catch {
    print(error)
}

var resultArray = [String]()
for item in json {
    if let content = item["content"] as? String {
        resultArray.append(content)
    }
}

print("resultArray: \(resultArray)")