How I find among all pairs a
and b
with a "least common multiple" LCM(a,b) = 498960 and a "greatest common divisor" GDM(a, b) = 12 a pair with minimum sum a + b
?
I solved this with O(n^2) time:
public class FindLcmAndGcdClass {
private int findGcd(int a, int b) {
if (a % b == 0) {
return b;
}
return findGcd(b, a % b);
}
private int findLcm(int a, int b, int gcd) {
return (a * b) / gcd;
}
private void run() {
int minSum = Integer.MAX_VALUE;
int foundNumberOne = 0;
int foundNumberTwo = 0;
for (int i = 12; i <= 498960; i += 12) {
for (int j = i; j <= 498960; j += 12) {
int gcd;
if (i < j) {
gcd = findGcd(j, i);
} else {
gcd = findGcd(i, j);
}
int lcm = findLcm(i, j, gcd);
if (gcd == 12 && lcm == 498960 && i + j < minSum) {
minSum = i + j;
foundNumberOne = i;
foundNumberTwo = j;
}
}
}
System.out.println(minSum);
System.out.println(foundNumberOne);
System.out.println(foundNumberTwo);
}
public static void main(String[] args) {
var o = new FindLcmAndGcdClass();
o.run();
}
}
And it executes quite slowly! I guess the problem can be solved with Dynamic Programming. Can anyone help with more fast solution?
I am not sure if this question can be solved with dynamic programming, but I think of a solution with time complexity O(sqrt(LCM * GCD))
.
It is well known that for any two integers a and b, LCM(a, b) * GCD(a, b) = a * b
. Therefore, you can first calculate the product of the gcd and lcm, (which is 5987520 in this question). Then for all its factors under sqrt(LCM * GCD)
, let a
be one of the factors, then b = LCM * GCD / a
. Test if gcd(a, b) = the required gcd, if so calculate the sum a + b
, then find the minimum among the sums, and you are done.