【哈希表】

char FirstNotRepeatingChar(char* pString)
{
    // invalid input
    if(!pString)
        return 0;

    // get a hash table, and initialize it
    const int tableSize = 256;                    //字符1个字节8位
    unsigned int hashTable[tableSize];          //unsigned？？
    for(unsigned int i = 0; i < tableSize; ++ i)
        hashTable[i] = 0;                        //为什么要置0？

    // get the how many times each char appears in the string
    char* pHashKey = pString;
    while(*(pHashKey) != '\0')
        hashTable[*(pHashKey++)] ++;

    // find the first char which appears only once in a string
    pHashKey = pString;
    while(pHashKey)
    {
        if(hashTable[*pHashKey] == 1)
            return *pHashKey;

        pHashKey++;
    }

    // if the string is empty
    // or every char in the string appears at least twice
    return 0;
}