OC、Swift 将数字转换成字母，字母转换成数字

OC 的实现方式：

NSString *a_String  = @"A";
NSInteger a_int     = [a_String characterAtIndex:0];//65

int b_int           = 66;
NSString *b_String  = [NSString stringWithFormat:@"%c ",b_int]; // B

Swift 的实现方式：

var a_String:String = "A"
var a_int:UInt16    = (a_String as NSString).character(at: 0)

var b_int:UInt16    = 66;
var b_string:String = String(format: "%c", b_int)