iOS 找出字符串string里所有子字符串substring的位置

- (NSArray<NSNumber *> *)getLocationsOfSubstring:(NSString *)substring atString:(NSString *)string {
    if (substring.length==0 || string.length==0) {
        return nil;
    }
    
    NSRange range = [string rangeOfString:substring];
    if (range.location == NSNotFound) {
        return nil;
    }
    
    NSMutableArray<NSNumber *> *locations = @[].mutableCopy;
    
    while (range.location != NSNotFound) {
        NSInteger location = range.location;
        if (locations.lastObject) {
            location += locations.lastObject.integerValue+substring.length;
        }
        [locations addObject:[NSNumber numberWithInteger:location]];
        
        string = [string substringFromIndex:NSMaxRange(range)];
        if (string.length == 0) {
            break;
        }
        
        range = [string rangeOfString:substring];
    }
    
    return locations.copy;
}