C++ to find the most characters and times in the string the time complexity is less than O of n^2

Given the string "aabbbcddddeeffffghijklmnopqrst", program to find the most characters and times, requiring a time complexity less than O(n^2)

/********************************************************
Copyright (C), 2016-2017,
FileName: main9
Author: woniu201
Description: Find the number of occurrences of a character in a string 
********************************************************/
#include "stdafx.h"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void search(char* pData, int len)
{
 char counts[1024] = {0}; // The number of times raw data is stored as an index 
 char bufMax[1024] = {0}; // Used to store characters that appear most often 
 int max = 0;  // The character that appears most often 
 for (int i=0; i<len; i++)
 {
 counts[pData[i]] ++;
 }
 for (int i=0; i<1024; i++)
 {
 if (counts[i] > max)
 {
 max = counts[i];
 bufMax[0] = i;
 }else if ((counts[i] == max) && (counts[i] !=0))
 {
 bufMax[strlen(bufMax)] = i;
 }
 }
 printf(" The characters that appear most are: ");
 for (int i=0; i<strlen(bufMax); i++)
 {
 printf("%c ", bufMax[i]);
 }
 printf("\n");
 printf(" The number of characters that appear most: %d\n", max);
}
int main()
{
 char* srcData = "aabbbcddddeeffffghijklmnopqrst";
 search(srcData, strlen(srcData));
 getchar();
 return 1;
}

conclusion