Twitter OA prepare: Rational Sum

In mathematics, a rational number is any number that can be expressed in the form of a fraction p/q , where p & q are two integers, and the denominator q is not equal to zero. Hence, all integers are rational numbers  where denominator, in the most reduced form, is equal to 1.
You are given a list of N rational number, {a1/b1, a2/b2, ..., aN/bN}. Print the sum ( = a1/b1 + a2/b2 + ... + aN/bN = num/den) in the most reduced form.
Input
The first line of input contains an integer, N, the number of rational numbers.  N lines follow. ithline contains two space separated integers, ai bi, where aiis the numerator and bi is the denominator for the ith rational number.
Output
You have to print two space separated integers, num den, where num and den are numerator and denominator of the sum respectively.
Constraints
1 <= N <= 15
1 <= ai <= 10
1 <= bi <= 10
Notes
Make sure the sum displayed as output is in the most reduced form.
If sum is an integer, you have to print 1 as denominator.
Sample Input
4
4 2
2 4
2 4
2 3
Sample Output
11 3

Explanation
Sum is 4/2 + 2/4 + 2/4 + 2/3 = (24 + 6 + 6 + 8)/12 = 44/12 = 11/3. So you have to print "11 3", which is the most reduced form.

Below is the syntax highlighted version of Rational.java from §9.2 Symbolic Methods. 摘自http://introcs.cs.princeton.edu/java/92symbolic/Rational.java.html

 /*************************************************************************
  *  Compilation:  javac Rational.java
  *  Execution:    java Rational
  *
  *  Immutable ADT for Rational numbers.
  *
  *  Invariants
  *  -----------
  *   - gcd(num, den) = 1, i.e, the rational number is in reduced form
  *   - den >= 1, the denominator is always a positive integer
  *   - 0/1 is the unique representation of 0
  *
  *  We employ some tricks to stave of overflow, but if you
  *  need arbitrary precision rationals, use BigRational.java.
  *
  *************************************************************************/

 public class Rational implements Comparable<Rational> {
     private static Rational zero = new Rational(0, 1);

     private int num;   // the numerator
     private int den;   // the denominator

     // create and initialize a new Rational object
     public Rational(int numerator, int denominator) {

         // deal with x/0
         //if (denominator == 0) {
         //   throw new RuntimeException("Denominator is zero");
         //}

         // reduce fraction
         int g = gcd(numerator, denominator);
         num = numerator   / g;
         den = denominator / g;

         // only needed for negative numbers
         if (den < 0) { den = -den; num = -num; }
     }

     // return the numerator and denominator of (this)
     public int numerator()   { return num; }
     public int denominator() { return den; }

     // return double precision representation of (this)
     public double toDouble() {
         return (double) num / den;
     }

     // return string representation of (this)
     public String toString() {
         if (den == 1) return num + "";
         else          return num + "/" + den;
     }

     // return { -1, 0, +1 } if a < b, a = b, or a > b
     public int compareTo(Rational b) {
         Rational a = this;
         int lhs = a.num * b.den;
         int rhs = a.den * b.num;
         if (lhs < rhs) return -1;
         if (lhs > rhs) return +1;
         return 0;
     }

     // is this Rational object equal to y?
     public boolean equals(Object y) {
         if (y == null) return false;
         if (y.getClass() != this.getClass()) return false;
         Rational b = (Rational) y;
         return compareTo(b) == 0;
     }

     // hashCode consistent with equals() and compareTo()
     public int hashCode() {
         return this.toString().hashCode();
     }

     // create and return a new rational (r.num + s.num) / (r.den + s.den)
     public static Rational mediant(Rational r, Rational s) {
         return new Rational(r.num + s.num, r.den + s.den);
     }

     // return gcd(|m|, |n|)
     private static int gcd(int m, int n) {
         if (m < 0) m = -m;
         if (n < 0) n = -n;
         if (0 == n) return m;
         else return gcd(n, m % n);
     }

     // return lcm(|m|, |n|)
     private static int lcm(int m, int n) {
         if (m < 0) m = -m;
         if (n < 0) n = -n;
         return m * (n / gcd(m, n));    // parentheses important to avoid overflow
     }

     // return a * b, staving off overflow as much as possible by cross-cancellation
     public Rational times(Rational b) {
         Rational a = this;

         // reduce p1/q2 and p2/q1, then multiply, where a = p1/q1 and b = p2/q2
         Rational c = new Rational(a.num, b.den);
         Rational d = new Rational(b.num, a.den);
         return new Rational(c.num * d.num, c.den * d.den);
     }

     // return a + b, staving off overflow
     public Rational plus(Rational b) {
         Rational a = this;

         // special cases
         if (a.compareTo(zero) == 0) return b;
         if (b.compareTo(zero) == 0) return a;

         // Find gcd of numerators and denominators
         int f = gcd(a.num, b.num);
         int g = gcd(a.den, b.den);

         // add cross-product terms for numerator
         Rational s = new Rational((a.num / f) * (b.den / g) + (b.num / f) * (a.den / g),
                                   lcm(a.den, b.den));

         // multiply back in
         s.num *= f;
         return s;
     }

     // return -a
     public Rational negate() {
         return new Rational(-num, den);
     }

     // return a - b
     public Rational minus(Rational b) {
         Rational a = this;
         return a.plus(b.negate());
     }

     public Rational reciprocal() { return new Rational(den, num);  }

     // return a / b
     public Rational divides(Rational b) {
         Rational a = this;
         return a.times(b.reciprocal());
     }

     // test client
     public static void main(String[] args) {
         Rational x, y, z;

         // 1/2 + 1/3 = 5/6
         x = new Rational(1, 2);
         y = new Rational(1, 3);
         z = x.plus(y);
         System.out.println(z);

         // 8/9 + 1/9 = 1
         x = new Rational(8, 9);
         y = new Rational(1, 9);
         z = x.plus(y);
         System.out.println(z);

         // 1/200000000 + 1/300000000 = 1/120000000
         x = new Rational(1, 200000000);
         y = new Rational(1, 300000000);
         z = x.plus(y);
         System.out.println(z);

         // 1073741789/20 + 1073741789/30 = 1073741789/12
         x = new Rational(1073741789, 20);
         y = new Rational(1073741789, 30);
         z = x.plus(y);
         System.out.println(z);

         //  4/17 * 17/4 = 1
         x = new Rational(4, 17);
         y = new Rational(17, 4);
         z = x.times(y);
         System.out.println(z);

         // 3037141/3247033 * 3037547/3246599 = 841/961
         x = new Rational(3037141, 3247033);
         y = new Rational(3037547, 3246599);
         z = x.times(y);
         System.out.println(z);

         // 1/6 - -4/-8 = -1/3
         x = new Rational( 1,  6);
         y = new Rational(-4, -8);
         z = x.minus(y);
         System.out.println(z);
     }

 }