JAVA code using hashmap

 public static int lonelyinteger(List<Integer> a) {
// Write your code here
    // Create a hashmap to store the frequency of each element
    HashMap<Integer, Integer> frequencyMap = new HashMap<>();

    // Populate the hashmap with element frequencies
    for (int num : a) {
        frequencyMap.put(num, frequencyMap.getOrDefault(num, 0) + 1);
    }

    // Find and return the element with a frequency of 1
    for (int key : frequencyMap.keySet()) {
        if (frequencyMap.get(key) == 1) {
            return key;
        }
    }
   return -1;
}

}