[Codility] Leader - Dominator / Java

1. 문제

An array A consisting of N integers is given. The dominator of array A is the value that occurs in more than half of the elements of A.

For example, consider array A such that

A[0] = 3 A[1] = 4 A[2] = 3 A[3] = 2 A[4] = 3 A[5] = -1 A[6] = 3 A[7] = 3

The dominator of A is 3 because it occurs in 5 out of 8 elements of A (namely in those with indices 0, 2, 4, 6 and 7) and 5 is more than a half of 8.

Write a function

class Solution { public int solution(int[] A); }

that, given an array A consisting of N integers, returns index of any element of array A in which the dominator of A occurs. The function should return −1 if array A does not have a dominator.

For example, given array A such that

A[0] = 3 A[1] = 4 A[2] = 3 A[3] = 2 A[4] = 3 A[5] = -1 A[6] = 3 A[7] = 3

the function may return 0, 2, 4, 6 or 7, as explained above.

Write an efficient algorithm for the following assumptions:

• N is an integer within the range [0..100,000];
• each element of array A is an integer within the range [−2,147,483,648..2,147,483,647].

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2. 코드

import java.util.*;

class Solution {
    public int solution(int[] A) {
    
        HashMap<Integer, Integer> map = new HashMap<>();
        
        for(int i=0; i<A.length; i++) {
            map.put(A[i], map.getOrDefault(A[i], 0) + 1);
        }
        
        int max = 0;
        int maxKey = 0;
        for(Integer key : map.keySet()) {
            int count = map.get(key);
            if(count > max) {
                max = count;
                maxKey = key;
            }
        }
        
        if(max <= A.length / 2)
            return -1;
        
        for(int i=0; i<A.length; i++) {
            if(A[i] == maxKey)
                return i;
        }
        
        return -1;
    }
}