CareerCup

typedef enum{false, true} bool;

int __atoi__(const char *str)
{
        bool isNeg = false;
        if(str[0] == '-')isNeg = true;
        int result = 0;
        int len = strlen(str);
        int i;

        for(i=isNeg; i<len; i++)
        {
                int digit = str[i]-'0';
                if(isNeg)
                {
                        if(result <(INT_MIN+digit)/10)return INT_MIN;
                        else result = result*10-digit;
                }
                else
                {
                        if(result > (INT_MAX-digit)/10)return INT_MAX;
                        else result = result*10 + digit;
                }
        }

        return result;
}

Answer to last part

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
using namespace std;

int main()
{
    string a="3";
    string b="65";
    int sum=atoi(a.c_str())+atoi(b.c_str());

    char Ssum[sizeof(sum)];
    itoa(sum,Ssum,10);
    string s=Ssum;
    cout<<s;
    int d;
    cin>>d;
    return 0;
}

sorry, the last part was multiply instead of add.

typedef enum{false, true} bool;

int __atoi__(const char *str)
{
        bool isNeg = false;
        if(str[0] == '-')isNeg = true;
        int result = 0;
        int len = strlen(str);
        int i;

        for(i=isNeg; i<len; i++)
        {
                int digit = str[i]-'0';
                if(isNeg)
                {
                        if(result <(INT_MIN+digit)/10)return INT_MIN;//Underflow. So return Min. int value...
                        else result = result*10-digit;
                }
                else
                {
                        if(result > (INT_MAX-digit)/10)return INT_MAX;//Overflow. So return Max. int value...
                        else result = result*10 + digit;
                }
        }

        return result;
}