NSString * str = @"abcdefg";
NSMutableString * string = [[NSMutableString alloc]initWithCapacity:str.length];
int j = (int)str.length;
for (int i = j - 1; i >= 0; i--) {
[string appendFormat:@"%c", [str characterAtIndex:i]];
}
NSLog(@"string____%@",string);