137.只出现一次的数字II

class Solution {
    public int singleNumber(int[] nums) {
        Map<Integer,Integer> map = new HashMap<>();
        for(int num : nums){
            Integer count = map.get(num);
            if(count == null){
                count = 1;
            }else{
                ++count;
            }
            map.put(num,count);
        }
        for(Integer val:map.keySet())
        {
            if(map.get(val) == 1){
                return val;
            }
        }
        return 0;        
    }
}