一道题目

given a string ,return the longest substring that contains at most two characters.

extern "C" char *SubStringWithAtMost2Chars(char * pStr, int len)
{
    if(len <= 2)
        return pStr;

    char ch1, ch2;
    int ch1Index= -1, ch1Cnt = 0, ch2Cnt = 0;
    char lastCh1 = '0', lastCh2 = '0';
    int lastCh1Index = -1,lastCh2Index = -1, lastCh1Cnt = 0, lastCh2Cnt = 0;

    for(int i = 0;i< len; i++)
    {
        if(lastCh1Cnt == 0)
        {
            lastCh1 = pStr[i];
            lastCh1Index = i;
            lastCh1Cnt ++;
        }
        else
        {
            if(lastCh2Cnt == 0)
            {
                if(pStr[i] == lastCh1)
                {
                    lastCh1Cnt++;
                }
                else
                {
                    lastCh2 = pStr[i];
                    lastCh2Cnt++;
                    lastCh2Index = i;
                }
            }
            else
            {
                if(pStr[i] == lastCh2)
                {
                    lastCh2Cnt++;
                }
                else
                {

                    if((lastCh1Cnt + lastCh2Cnt) > (ch1Cnt + ch2Cnt))
                    {
                        ch1 = lastCh1;
                        ch2 = lastCh2;
                        ch1Index = lastCh1Index;
                        ch1Cnt = lastCh1Cnt;
                        ch2Cnt = lastCh2Cnt;
                    }
                    lastCh1 = lastCh2;
                    lastCh1Index = lastCh2Index;
                    lastCh1Cnt = lastCh2Cnt;

                    //below is missed
                    lastCh2Cnt = 1;
                    lastCh2Index = i;
                    lastCh2 = pStr[i];
                }
            }
        }
    }
    if((lastCh1Cnt + lastCh2Cnt) > (ch1Cnt + ch2Cnt))
    {
        ch1 = lastCh1;
        ch2 = lastCh2;
        ch1Index = lastCh1Index;
        ch1Cnt = lastCh1Cnt; //missed
        ch2Cnt = lastCh2Cnt; //missed
    }

    char *pSubStr = (char *) malloc(ch1Cnt + ch2Cnt + 1);
    memset(pSubStr, 0, ch1Cnt + ch2Cnt + 1);
    memcpy(pSubStr,pStr+ ch1Index, ch1Cnt + ch2Cnt);
}