#include <limits.h>

char findFirstNonRepeating(char* str)
{
    char count[26] = {0}; // O(1)
    char index[26] = {-1}; // O(1)

    // scan the string - O(n)
    for (int i = 0; str[i] != '\0'; ++i)
    {
        char c = str[i];

        ++count[c - 'A'];

        // if this character has not been found yet, mark it as found
        if (index[c - 'A'] == -1)
            index[c - 'A'] = i;
    }

    // find the minimum index at which count is 1. This is O(1).
    int minIndex = INT_MAX;
    for (int i = 0; i < 26; ++i)
    {
        if (index[i] != -1)
        {
            if (count[i] == 1 && index[i] < minIndex)
                minIndex = index[i];
        }
    }

    if (minIndex == INT_MAX)
        return '\0';
    else
        return str[minIndex];
}