128. Longest Consecutive Sequence

竟然是hard, 解法是比较巧妙但很好理解，没有复杂算法

class Solution {
    public int longestConsecutive(int[] nums) {
        if (nums == null || nums.length == 0){
            return 0;
        }
        Set<Integer> set = new HashSet<>();
        for (int num : nums){
            set.add(num);
        }
        //[100,4,200,1,3,2]
        //longest = 0
        //num : 100
        //down = 99
        //up = 101
        int longest = 0;
        for (int i = 0; i < nums.length; i++){
            int down = nums[i] - 1;
            while (set.contains(down)){
                set.remove(down);
                down--;
            }
            int up = nums[i] + 1;
            while (set.contains(up)){
                set.remove(up);
                up++;
            }
            //actually up - 1 - (down + 1) + 1
            longest = Math.max(longest, up - down - 1);
        }
        return longest;
    }
}