class  {
    public int firstMissingPositive(int[] nums) {
        if (nums==null){
              return 0;
           }
           Set<Integer> set=new HashSet<>();
           for (int num:nums){
               set.add(num);
           }
           int i=1;
           while(set.contains(i)){
               i++;
           }
           return i;
    }
}