136. Single Number - LeetCode

public int singleNumber(int[] nums) {
    Map<Integer, Integer> countMap = new HashMap<>();
    for (int i=0; i<nums.length; i++) {
        Integer count = countMap.get(nums[i]);
        if (count == null) {
            count = 0;
        }
        countMap.put(nums[i], count + 1);
    }
    int num = -1;
    for (Map.Entry<Integer, Integer> entry : countMap.entrySet()) {
        if (entry.getValue() == 1) {
            num = entry.getKey();
            break;
        }
    }
    return num;
}