UVa Online Judge 100 - The 3n + 1 Problem and Solution in C++

Problems in Computer Science are often classifed as belonging to a certain class of problems (e.g.,
NP, Unsolvable, Recursive). In this problem you will be analyzing a property of an algorithm whose
classification is not known for all possible inputs.

Consider the following algorithm:

1. input n
2. print n
3. if n = 1 then STOP
4. if n is odd then n − 3n + 1
5. else n − n/2
6. GOTO 2

Given the input 22, the following sequence of numbers will be printed

22 11 34 17 52 26 13 40 20 10 5 16 8 4 2 1

It is conjectured that the algorithm above will terminate (when a 1 is printed) for any integral input
value. Despite the simplicity of the algorithm, it is unknown whether this conjecture is true. It has
been verifed, however, for all integers n such that 0 < n < 1; 000; 000 (and, in fact, for many more
numbers than this.)

Given an input n, it is possible to determine the number of numbers printed before and including
the 1 is printed. For a given n this is called the cycle-length of n. In the example above, the cycle
length of 22 is 16.

For any two numbers i and j you are to determine the maximum cycle length over all numbers
between and including both i and j.

Input

The input will consist of a series of pairs of integers i and j, one pair of integers per line. All integers
will be less than 10,000 and greater than 0.
You should process all pairs of integers and for each pair determine the maximum cycle length over
all integers between and including i and j.
You can assume that no operation overflows a 32-bit integer.

Output

For each pair of input integers i and j you should output i, j, and the maximum cycle length for
integers between and including i and j. These three numbers should be separated by at least one space
with all three numbers on one line and with one line of output for each line of input. The integers i
and j must appear in the output in the same order in which they appeared in the input and should be
followed by the maximum cycle length (on the same line).

Sample Input

1 10
100 200
201 210
900 1000

Sample Output

1 10 20
100 200 125
201 210 89
900 1000 174


Solution in C++

#include <cstdio>
#include <algorithm>
#define long long LL

using namespace std;

int main ()
{
    int i, j;

    while ( scanf ("%d %d", &i, &j) != EOF ) {

        int temp_i = i;
        int temp_j = j;

        if ( i > j ) swap (i, j);

        int max_cycle_length = 0;
        int cycle_length;

        while ( i <= j ) {
            unsigned int n = i;
            cycle_length = 1;

            while ( n != 1 ) {
                if ( n % 2 == 1 ) n = 3 * n + 1;
                else n /= 2;
                cycle_length++;
            }

            if ( cycle_length > max_cycle_length )
                max_cycle_length = cycle_length;

            i++;
        }

        printf ("%d %d %d\n", temp_i, temp_j, max_cycle_length);
    }

    return 0;
}