NumberAppearOnce.java

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

public class NumberAppearOnce {
    public static int bruteapproach(int[] arr) {
        for (int i = 0; i < arr.length; i++) {
            int num = arr[i];
            int cnt = 0;
            for (int j = 0; j < arr.length; j++) {
                if (num == arr[j]) {
                    cnt++;
                }
            }
            if (cnt == 1) {
                return num;
            }
        }
        return -1;
    }
    public static int better1(int[] arr) { // IMPLEMENTING HASHMAP USING ARRAY DS
        int maxi = arr[0];
        for (int i = 1; i < arr.length; i++) {
            if (maxi < arr[i]) {
                maxi = arr[i];
            }
        }
        int []hash = new int[maxi+1];
        for (int i = 0; i < arr.length; i++) {
            hash[arr[i]]++;
        }
        for (int i = 0; i < arr.length; i++) {
            if (hash[arr[i]] == 1) {
                return arr[i];
            }
        }
        return -1;
    }
    public static int better2(int[] arr) { //IMPLEMENTING HASHMAP USING MAP DS
        HashMap<Integer, Integer> mpp = new HashMap<>();
        for (int i = 0; i < arr.length; i++) {
            int value = mpp.getOrDefault(arr[i], 0);
            mpp.put(arr[i], value + 1);
        }
        for (Map.Entry<Integer, Integer> it : mpp.entrySet()) {
            if (it.getValue() == 1) {
                return it.getKey();
            }
        }
        return -1;
    }

    public static int optimal(int[] arr) {
        int xor = 0;
        for (int i = 0; i < arr.length; i++) {
            xor ^= arr[i];
        }
        return xor;
    }
    public static void main(String[] args) {
        int[] arr = {1,1,2,2,3,4,4,5,6,5,6,8,8};
        System.out.println(optimal(arr));
    }
}