今天看到这个“Objective-C分割NSString”，有人用正则表达式进行解答，感觉很特别。所以就把以前遇到的字符串反转的问你拿出来讨论讨论，可不可以用正则表达式来解决呢？
下面是个最简单的实现的如有什么效率问题，边界问题也希望多给意见。
/**** NSString+Reverse.h ****/
#import <Foundation/Foundation.h>
@interface NSString (Reverse)
- (NSString *)stringByReversed;
@end
/**** NSString+Reverse.m ****/
#import "NSString+Reverse.h"
@implementation NSString (Reverse)
- (NSString *)stringByReversed
{
NSMutableString *s = [NSMutableString string];
for (NSUInteger i=self.length; i>0; i--) {
[s appendString:[self substringWithRange:NSMakeRange(i-1, 1)]];
}
return s;
}
@end
分割线 根据@ParagonLight同学的回答我用如下代码做了个测试：
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init]; [dateFormatter setDateFormat:@"ss.SSSS"]; NSLog(@"S %@", [dateFormatter stringFromDate:[NSDate date]]); NSString *reversed = [string stringByReversed]; NSLog(@"E %@", [dateFormatter stringFromDate:[NSDate date]]);
测试用的字符串长度为：8970
记录了10次结果，我制作了一个图表（算法1为我问题中的；算法2为@ParagonLight同学的）
只看代码的话算法2可以提高一半的效率，之所以花的时间比算法1高出许多，是因为每次循环都会生成一个新string对象，所耗费时间确实超乎我的预想了。
分割线由于算法1和算法3相差很小，为了让结果更明显，修改了测试用例，将测试字符串长度放大100倍，现在测试字符串长度为：897000
@ParagonLight同学的算法3的性能提升还是很明显的
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I used the stringByReplacingCharactersInRange: method here. But this method actually creates a new string, so it doesn't feel like the most efficient approach. In fact, you can convert the string into a char array, then set two pointers to point to the head and tail of the array, and then exchange the pointed values in sequence until i>j.
I haven't been in contact with iOS for a long time, and I don't know much about its features. Please point out if there is anything wrong.
- (NSString *)stringByReversed { NSUInteger i = 0; NSUInteger j = self.length - 1; NSString *temp; NSString *newString = self; NSLog(@"%@", self); while (i < j) { temp = [newString substringWithRange:NSMakeRange(i, 1)]; newString = [newString stringByReplacingCharactersInRange:NSMakeRange(i, 1) withString:[self substringWithRange:NSMakeRange(j, 1)]]; newString = [newString stringByReplacingCharactersInRange:NSMakeRange(j, 1) withString:temp]; NSLog(@"%@",newString); i ++; j --; } NSLog(@"%@", newString); return newString; }Separating lineTry this one
-(NSString *)stringByReversed{ NSUInteger i = 0; NSUInteger j = self.length - 1; unichar characters[self.length]; while (i < j) { characters[j] = [self characterAtIndex:i]; characters[i] = [self characterAtIndex:j]; i ++; j --; } if(i == j) characters[i] = [self characterAtIndex:i]; return [NSString stringWithCharacters:characters length:self.length]; }Update: Algorithm 4Visual inspection indicates that the unichar array is out of bounds. Just malloc directly. . . But I have to say, this is no longer OC. . .
- (NSString *)stringByReversed{ uint64_t i = 0; uint64_t j = self.length - 1; // unichar characters[self.length]; unichar *characters = malloc(sizeof([self characterAtIndex:0]) * self.length); while (i < j) { characters[j] = [self characterAtIndex:i]; characters[i] = [self characterAtIndex:j]; i ++; j --; } if(i == j) characters[i] = [self characterAtIndex:i]; return [NSString stringWithCharacters:characters length:self.length]; }