Codility 22번 Dominator

Dominator

 

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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import java.util.*;

// you can write to stdout for debugging purposes, e.g.
// System.out.println("this is a debug message");

class Solution {
    public int solution(int[] A) {
        Stack<Integer> item = new Stack<>();
        for(int i=0; i<A.length; i++){
            if(item.isEmpty() || item.peek() == A[i]){
                item.push(A[i]);
            }else{
                item.pop();
            }
        }
        int cnt=0;
        int pos = -1;
        if(item.isEmpty()){
            return -1;
        }
        int num = item.pop();
        for(int i=0; i<A.length; i++){
            if(A[i] == num){
                if(pos == -1){
                    pos = i;
                }
                cnt++;
            }
        }
        if(A.length / 2 < cnt) {
            return pos;
        }else{
            return -1;
        }
        // write your code in Java SE 8
    }
}

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