//字符串反转
    NSString *str = @"abcedfghijklmnopqrstuvwxyz";
    NSMutableString *string = [NSMutableString stringWithCapacity:str.length];
    int j = (int)str.length;
    for (int i = j - 1; i>=0; i--) {
        [string appendFormat:@"%c",[str characterAtIndex:i]];
    }
    
    NSLog(@"string is-- %@",string);

string is-- zyxwvutsrqponmlkjihgfdecba