EquiLeader – Codility – Solution

Java solution to Codility EquiLeader problem (Lesson 8 – Leader) which scored 100%. The problem is to find the two adjoining sub-arrays with the same integer occurring in more than half the elements of each sub-array.

The strategy is:

• find the dominator of entire array with the same code as from the Dominator problem If no Dominator, no equi leaders can exist.
• keep running count of occurrences of dominator at each array index. That way, can calculate if there’s an equi leader pair at every index in given array.

[cc lang="java" escaped="true"]
package com.codility.lesson08.leader;

import java.util.HashMap;
import java.util.Map;
import java.util.Stack;

public class EquiLeader {
	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 equi leaders if stack is empty
		if(stack.isEmpty()) return 0;
		int candidate = stack.peek().intValue();
		int dominatorCount = 0;

		Map<Integer, Integer> dominatorMap = new HashMap<Integer, Integer>();
		for(int i=0; i<A.length; i++) { if(A[i] == candidate) { dominatorCount++; dominatorMap.put(i, dominatorCount); } } //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
		int equiLeaders = 0;
		if(dominatorCount > (A.length / 2)) {
			//find all equi leader sequences
			
			int lastCandidateOccurenceIndex = 0;
			int runningDominatorCount = 0;
			for(int i=0; i<A.length-1; i++) { if(A[i] == candidate) { lastCandidateOccurenceIndex = i; runningDominatorCount = dominatorMap.get(i).intValue(); } else if(dominatorMap.get(lastCandidateOccurenceIndex) != null) { runningDominatorCount = dominatorMap.get(lastCandidateOccurenceIndex).intValue(); } if(runningDominatorCount > (i+1)/2) {
					if((dominatorCount - runningDominatorCount) > (A.length - (i+1))/2 ) {
						equiLeaders++;
					}
				}
			}
		}
		return equiLeaders;
	}
}
[/cc]

TestNG test cases for this problem which all passed:

[cc lang="java" escaped="true"]
package test.com.codility.lesson08.leader;

import org.testng.Assert;
import org.testng.annotations.BeforeTest;
import org.testng.annotations.DataProvider;
import org.testng.annotations.Test;

import com.codility.lesson08.leader.EquiLeader;

public class EquiLeaderTests {
	private EquiLeader solution;
	
	@BeforeTest
	public void setUp() {
		solution = new EquiLeader();
	}

	@DataProvider(name = "test1")
	public Object [][] createData1() {
		return new Object [][] {
			new Object [] { new int [] { 4, 3, 4, 4, 4, 2 }, 2 },
			new Object [] { new int [] { 1, 5, 1, 5, 5, 5, 5, 5 }, 3 },
			new Object [] { new int [] { 1, 1, 1, 1, 1, 1 }, 5 },
			new Object [] { new int [] { 4, 4, 2, 5, 3, 4, 4, 4 }, 3 },
			
			new Object [] { new int [] { 1, 2, 3  }, 0 },
			new Object [] { new int [] { 1000000000  }, 0 },
			new Object [] { new int [] { 1000, 1  }, 0 },
			new Object [] { new int [] { 0, 0  }, 1 },
		};
	}

	@Test(dataProvider = "test1")
	public void verifySolution(int [] pA, int pExpected) {		
		Assert.assertEquals(solution.solution(pA), pExpected);
	}	
}
[/cc]