Java solution to Codility Dominator problem (Lesson 8 – Leader) which scored 100%. The problem is to find the value that occurs in more than half of the elements of a given integer array.
The strategy is:
- use a java.util.Stack and push duplicate values onto the stack to find a dominator candidate
- if encounter a number that doesn’t exist on top of stack, pop the stack
- if the stack is not empty after going through all the array elements, then we have a candidate (not necessarily a dominator)
- go through array again to see if the dominator value occurrs more than n/2 times
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import java.util.Stack;
public class Dominator {
public int solution(int[] A) {
Stack stack = new Stack();
for(int i=0; i<A.length; i++) {
if(stack.isEmpty()) {
stack.push(A[i]);
}
else {
if(stack.peek().intValue() == A[i]) {
stack.push(A[i]);
}
else {
stack.pop();
}
}
}
//no candidate if stack is empty
if(stack.isEmpty()) return -1;
int candidate = stack.peek().intValue();
int count = 0;
int candidateIndex = -1;
for(int i=0; i<A.length; i++) {
if(A[i] == candidate) {
count++;
if(candidateIndex < 0) { candidateIndex = i; //only store index of first occurence of candidate } } } //works for even and odd number of A elements //e.g. if A.length = 4, count needs to be > 2
//e.g. if A.length = 5, count needs to be > 2
if(count > (A.length / 2)) return candidateIndex;
return -1; //no dominator found
}
}
TestNG test cases for this problem which all passed:
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import org.testng.Assert;
import org.testng.annotations.BeforeTest;
import org.testng.annotations.DataProvider;
import org.testng.annotations.Test;
import com.codility.lesson08.leader.Dominator;
public class DominatorTests {
private Dominator solution;
@BeforeTest
public void setUp() {
solution = new Dominator();
}
@DataProvider(name = "test1")
public Object [][] createData1() {
return new Object [][] {
new Object [] { new int [] { 3, 4, 3, 2, 3, -1, 3, 3 }, 0 },
new Object [] { new int [] { 100, 100, 100, 100, 100, 100 }, 0 },
new Object [] { new int [] { 100, -1000, 100, -1000, -1000 }, 1 },
new Object [] { new int [] { 1, 2, 3 }, -1 },
new Object [] { new int [] { 1000000000 }, 0 },
new Object [] { new int [] { 1000, 1 }, -1 },
new Object [] { new int [] { 0, 0 }, 0 },
};
}
@Test(dataProvider = "test1")
public void verifySolution(int [] pA, int pExpected) {
Assert.assertEquals(solution.solution(pA), pExpected);
}
}