keypad.txt

/*
 2 - ABC, 3- DEF, 4- GHI, 5-JKL, 6-MNO, 7- PQR, 8- STU, 9- VWXYZ. Each number represents the set of variables. When we input a number, it should be replaced by all possible conbination of the corresponding charactors. 
 eg : if we enter 234, OUTPUT would be ADG,ADH....CFI.
*/

#include <iostream>
using namespace std;

string pad[] = {"0", "1", "ABC", "DEF", "GHI", "JKL", "MNO", "PQR", "STU", "VWXYZ"};

/*The function below output all the possible combination of the key pad, given a string of numbers.*/
void keypad(string str, int index){
    if (index==str.length())
        cout<<str<<endl;
    else{
        int num = str[index]-'0';
        for (int i=0; i<pad[num].length(); ++i){
            str[index] = pad[num][i];
            keypad(str,index+1);
            str[index] = num-'0';
        }
    }
}

int main(){
    string str;
    cout<<"Input an int string:"<<endl;
    cin>>str;
    for (int i=0; i<str.length(); ++i){
        if (str[i]>'9'||str[i]<'0'){
            cout<<"wrong input."<<endl;
            return -1;
        }
    }
    /*The next one line only to show the difference between str[index] and str[index]-'0'*/
    cout<<"str[0] is: "<<str[0]<<", while str[0]-'0' is: "<<str[0]-'0';
    keypad(str,0);
    return 0;
}
REMARK:
1. Hard question. Had no idea.
2. Note on line 16 and 20, we use str[index]-'0' and num-'0' so as to convert a char into an int. EXAMPLE: int('4')=52, int('4'-'0')=4. VERY IMPORTANT!!!!!!!! ERROR PRONE!!!!