class Solution {
    // approach: XOR (a^a = 0)
    public int singleNumber(int[] nums) {
        int n = nums.length;
        Map<Integer, Integer> map = new HashMap<>();
        for(int ele : nums)
        {
            map.put(ele , map.getOrDefault(ele, 0) + 1);
        }

        int ans = 0;
        for(Map.Entry<Integer,Integer> entry : map.entrySet())
        {
            int key = entry.getKey();
            int value = entry.getValue();

            if(value == 1)
            {
                ans = key;
                break;
            }
        }
        return ans;
    }
   
    // approach: XOR (a^a = 0)
    public int singleNumber(int[] nums) {
        int n = nums.length;
        int xor = 0;
        for(int ele : nums)
        {
            xor = xor ^ ele;
        }
        return xor;
    }
}