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    Java using frequency map:

    public static int lonelyinteger(List<Integer> a) {
            if (a.size() == 1) return a.get(0);
            
            Map<Integer, Integer> map = new HashMap<>();
            
            for (int i : a) {
                map.put(i, map.getOrDefault(i, 0) + 1);
            }
    
            int result = 0;
            
            for (Map.Entry<Integer,Integer> e : map.entrySet()) {
                if (e.getValue() == 1) result = e.getKey();
            }
            
            return result;
        }