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Arrays_MajorityElement.java
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Arrays_MajorityElement.java
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import java.io.IOException;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.HashMap;
import java.util.Map;
// https://practice.geeksforgeeks.org/problems/majority-element/0
// https://leetcode.com/problems/majority-element/
// https://www.geeksforgeeks.org/majority-element/
// https://www.youtube.com/watch?v=bg6r_fgtpMQ (BEST video tutorial)
// https://www.youtube.com/watch?v=0s3zqYaDInE (Using Bit masking)
class Arrays_MajorityElement {
public static void main(String[] args) throws IOException {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
int t = Integer.parseInt(br.readLine().trim());
while(t-- > 0) {
int n = Integer.parseInt(br.readLine().trim());
String[] str = br.readLine().trim().split("\\s+");
int[] input = new int[n];
for (int i = 0; i < input.length; i++) {
input[i] = Integer.parseInt(str[i]);
}
//System.out.println(getMajorityElement1(input)); // time complexity : O(n), space complexity : O(n)
System.out.println(getMajorityElement2(input)); // time complexity : O(n), space complexity : O(1)
}
}
// METHOD-1
private static int getMajorityElement1(int[] input) {
int majority = input.length / 2;
Map<Integer, Integer> map = new HashMap<>();
for(int num : input) {
map.put(num, map.getOrDefault(num, 0) + 1);
if(map.get(num) > majority) return num;
}
return -1;
}
// METHOD-2 (Boyer-Moore Voting Algorithm)
private static int getMajorityElement2(int[] input) {
int probableMajorityCandidate = findProbableMajorityCandidate(input);
return isMajority(probableMajorityCandidate, input) ? probableMajorityCandidate : -1;
}
private static int findProbableMajorityCandidate(int[] input) {
int count = 1;
int currentMajorityCandidate = input[0];
for(int i = 1; i < input.length; i++) {
if(count == 0) currentMajorityCandidate = input[i];
count += (currentMajorityCandidate == input[i]) ? 1 : -1;
}
return currentMajorityCandidate;
}
private static boolean isMajority(int probableMajorityCandidate, int[] input) {
int majority = input.length / 2;
int count = 0;
for (int num : input) {
if(num == probableMajorityCandidate) count++;
}
return (count > majority);
}
}