NSMutableString可变字符串

NSMutableString继承自NSString

插入字符串

NSMutableSring *str = [NSMutableString stringWithString:@"abc"];
[str insertString:@"def" atIndex:0];

删除字符串

NSMutableSring *str = [NSMutableString stringWithString:@"abc"];
[str deleteCharactersInRage:NSMakeRage(1,2)];

替换字符串

NSMutableSring *str = [NSMutableString stringWithString:@"abc"];
[str replaceCharactersInRage:NSMakeRage(1,2) withString:@"def"];