Re: any suggestion to improve the following code?

Hello, the following code is an implementation computing the 3n+1
problem, specifically maximum number of steps to complete for any
integer range. The algorithm aims to be as efficient/fast as possible.

One thing I am unsatisfied of is the use of hash_map (deprecated but
supported by most C++ vendors). Is there a portable hash map
implementation? I am tempted to use unordered_map with boost::hash. I
didn't use std::map because std::map lookup/insertion is O(lgN) while
hash map is O(1). What's your opinion?

Comments about style etc are
welcome as well regarding other aspects of my implementation.

Fei

#include <iostream>
#include <fstream>
#include <algorithm>

using namespace std;

#include <ext/hash_map>
using namespace __gnu_cxx;

#include <boost/filesystem/convenience.hpp>

hash_map<unsigned int, unsigned int, hash<unsigned int> > steps;

// Computes number of steps for number n according
// to Collatz Conjecture (3n+1 problem)
// http://en.wikipedia.org/wiki/Collatz_conjecture
//
// To speed things up, results are book kept, saved/restored
// when program starts/finishes.
//
// In the recursively computing function, the steps to finish a
// a number is always memorized and retrieved on demand.
unsigned int compute_steps(int n){
    // shortcut to retrieve memorized steps[n]
    if(steps[n])
        return steps[n];

    if(n == 1) return 1;
    if(n%2)
        n = 3*n+1;
    else
        n = n/2;
    cout << ' ' << n;
    // shortcut to memorize steps[n]
    steps[n] = compute_steps(n);
    return steps[n] + 1;
}

int main(){
    boost::filesystem::path file("record_h.txt");

    unsigned int two[2];
    if(exists(file)){
        ifstream inf("record_h.txt", ios::binary);
        while(inf.read((char *)two, 2*sizeof(unsigned int)))
            steps[two[0]] = two[1];
        inf.close();
    }

    int i, j;
    // It's not as easy as it seems to safely read integers from cin
    // The following trick makes sure a pair of integers are read in safely
    while(!(cin >> i >> j)) {
        cout << i << ' ' << j << endl;
        cin.clear();
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
    }
    assert(i != 0 && j >= i);

    // compute steps for each l between i and l and prints its value
    for(int l = i; l <= j; l ++){
        cout << l << ": ";
        steps[l] = compute_steps(l);
        cout << '\n';
    }

    // compute some statitics of the result, the largest number n
steps[n] is computed
    // the number of 0s from steps to steps+rbegin

    unsigned int max_step = 0;
    ofstream ofs("record_h.txt", ios::binary);
    hash_map<unsigned int, unsigned int, hash<unsigned int>


    while(it != steps.end()){
        two[0] = it->first;
        two[1] = it->second;
        max_step = (max_step > it->second) ? max_step : it->second;
        ofs.write((const char *)two, 2*sizeof(unsigned int));
        ++it;
    }
    ofs.close();
    cout << "maximum step: " << i << ' ' << j << ' ' << max_step << '\n';
}