最长连续序列

class  {
    public int longestConsecutive(int[] nums) {
         if (nums==null||nums.length==0){
               return 0;
           }
           int res=0;
        Set<Integer> set=new HashSet<>();
           for (int num:nums){
               set.add(num);
           }
           for (int num:nums){
               if (set.remove(num)){
                    int pre=num-1,next=num+1;
                    while (set.remove(pre)){--pre;}
                    while(set.remove(next)){++next;}
                    res=Math.max(res,next-pre-1);
               }
           }
           return res;
    }
}