appearOnce.java

import java.util.HashMap;

public class appearOnce {
    public static void main(String[] args) {
        int a[] = { 1, 1, 2, 2, 3, 3, 4, 50, 50, 65, 4, 65, 5 };// find ele which one appear once

        // first approach 
        HashMap<Integer, Integer> map = new HashMap<>();

        for (int num : a) {
            map.put(num, map.getOrDefault(num, 0) + 1);        
        }

        for (int num : map.keySet()) {            
            if (map.get(num) == 1) {
                System.out.println(" "+num);
            }
        }

        // second approach  

        // for (int i = 0; i < a.length; i++) {
        //     int count = 0;

        //     for (int j = 0; j < a.length; j++) {
        //         if (a[i] == a[j]) {
        //             count++;
        //         }
        //     }
        //     if (count == 1){
        //         System.out.print(a[i]);
        //     }
        // }

    }
}