Alexander.cc

#include <stdio.h>

int addPrefix(int N, int digit) {
    int multiplier = 0;
    // [1, 5]
    if(N <= 100000) {
        // [1, 3]
        if(N <= 1000) {
            //[1, 2]
            if(N <= 100) {
                //[1, 1]
                if(N <= 10) {
                    multiplier = 10;
                //[2, 2]
                } else {
                    multiplier = 100;
                }
            //[3, 3]
            } else {
                multiplier = 1000;
            }
        //[4, 4]
        } else if(N <= 10000) {
            multiplier = 10000;
        //[5, 5]
        } else {
            multiplier = 100000;
        }
    //[6, 7]
    } else if(N <= 10000000) {
        //[6, 6]
        if(N <= 1000000) {
            multiplier = 1000000;
        //[7, 7]
        } else {
            multiplier = 10000000;
        }
    //[8, 9]
    } else {
        //[8, 8]
        if(N <= 100000000) {
            multiplier = 100000000;
        //[9, 9]
        } else {
            multiplier = 1000000000;
        }
    }
    return N + digit * multiplier;
}

int main(void) {
    int i, j, k, N = 2;
    for(i = 1; i < 9; ++i, N *= 10) {
        for(j = 1; j < 50000000; ++j) {
            k = addPrefix(N, 9);
        }
    }
  return 0;
}

Daniel Frey.cc

#include <iostream>

int add_prefix( int number, int prefix )
{
    // without the above, the result of (X) would be "90"
    int tmp = 10;
    while( number >= tmp ) tmp *= 10;
    return number + prefix * tmp;
}

int main(void) {
    int i, j, N = 2, k;
    for(i = 1; i < 9; ++i, N *= 10) {
        for(j = 1; j < 50000000; ++j) {
            k = add_prefix(N, 9);
        }
    }
  return 0;
}

kkuryllo.cc

#include <stdio.h>
#include <math.h>

int addPrefix(int x, int addition) {
    int y = 1;
    while (y <= x)
    {
        y *= 10;
    }
    x += addition * y;
    return x;
}

int main(void) {
    int i, j, N = 2, k;
    for(i = 1; i < 9; ++i, N *= 10) {
        for(j = 1; j < 50000000; ++j) {
            k = addPrefix(N, 9);
        }
    }
  return 0;
}

WB.cc

#include <stdio.h>
#include <math.h>

int addPrefix(int N, int digit) {
    int noDigits = floor(log10(N)) + 1;
    //Add 9 in front
    return N + digit * pow(10, noDigits);
}

int main(void) {
    int i, j, N = 2, k;
    for(i = 1; i < 9; ++i, N *= 10) {
        for(j = 1; j < 50000000; ++j) {
            k = addPrefix(N, 9);
        }
    }
  return 0;
}