ASSCIICounter.h

#import "ASCIICounter.h"

static const int kASCIISymbolsCount = 256;

char* fillMappingArrayInSequence(char* str, int size, char* asciiMappingArray) {
  	  
    for (int currentSymbolPosition = 0; currentSymbolPosition < size; currentSymbolPosition++) {
        asciiMappingArray[str[currentSymbolPosition]]++;
    }
    return asciiMappingArray;
}

char mostFrequentCharacterFromMappingArray(char *array) {
    int maxCount = 0;
    char maxCountPosition = 0;
    for (int currentAsciiSymbol = 0; currentAsciiSymbol < kASCIISymbolsCount; currentAsciiSymbol++) {
        if (array[currentAsciiSymbol] > maxCount) {
            maxCount = array[currentAsciiSymbol];
            maxCountPosition = currentAsciiSymbol;
        }
    }
    return maxCountPosition;
}

char* mergeMappingArraysToFirst(char* array1, char* array2) {
    for (int currentAsciiSymbol = 0; currentAsciiSymbol < kASCIISymbolsCount; currentAsciiSymbol++) {
        array1[currentAsciiSymbol] = array1[currentAsciiSymbol] + array2[currentAsciiSymbol];
    }
    return array1;
}

char mostFrequentCharacter(char *str ,int size) {
    //Делим массив пополам
    int size1 = size / 2;
    int size2 = size - size1;
  
    __block char * asciiMappingArray1 = (char *)calloc(kASCIISymbolsCount, sizeof(char));
    __block char * asciiMappingArray2 = (char *)calloc(kASCIISymbolsCount, sizeof(char));
    
    //Создание параллельной очереди
    dispatch_queue_t queue = dispatch_queue_create("test char", DISPATCH_QUEUE_CONCURRENT);
    
    //Запуск счетчика по левой части строки
    dispatch_async(queue, ^{
        fillMappingArrayInSequence(str, size1, asciiMappingArray1);
    });
    
    //Запуск счетчика по правой части строки
    dispatch_async(queue, ^{
        fillMappingArrayInSequence(str + size1, size2, asciiMappingArray2);
    });
    
    //Синхронизируем и объединяем счетчики
    dispatch_barrier_sync(queue, ^{mergeMappingArraysToFirst(asciiMappingArray1, asciiMappingArray2);});
    
    char resultChar = mostFrequentCharacterFromMappingArray(asciiMappingArray1);
  
    free(asciiMappingArray1);
    free(asciiMappingArray2);
    
    return resultChar;
}

@implementation ASCIICounter
- (char) mostFrequentCharacter:(char *)str size:(int)size {
    return mostFrequentCharacter(str, size);
}
@end

main.m

ASCIICounter *counter = [[ASCIICounter alloc] init];
NSLog(@"%c", [counter mostFrequentCharacter:"abcabcfffffgggg" size:15]);