Swift3.0 中不能直接使用下标数字进行字符串截取，只能使用String.Index来做位置索引，要想实现截取功能首先得获取到String.Index;

let mainString = "0123456789"
let index = mainString.index(mainString.endIndex,offsetBy: -1)
print(mainString.substring(from: index))
print(mainString.substring(to: index))

打印结果：012345678

let mainString = "0123456789"
let index = mainString.index(mainString.startIndex,offsetBy: 1)
print(mainString.substring(from: index))
print(mainString.substring(to: index))

打印结果：0