【leetcode】 外观数列

char * countAndSay(int n){
    int i,j,cnt,len;    
    char buffer[100]={0};
    char* pre = (char*)calloc(10000,sizeof(char));
    sprintf(pre,"%d",1);
    for (i=2; i<=n; i++)
    {
        cnt=0;
        len=strlen(pre);
        char* retStr = (char*)calloc(10000,sizeof(char));
        for(j=0; j<len; j++)
        {
            cnt++;
            if (j == strlen(pre)-1 || pre[j] != pre[j+1])
            {            
                sprintf(buffer,"%d",cnt);
                buffer[1]=pre[j];
                strcat(retStr,buffer);
                cnt=0;
            }
        }
        pre=retStr;
    }
    return pre;
}