【Leetcode】Longest Consecutive Sequence

题目链接：https://leetcode.com/problems/longest-consecutive-sequence/
题目：

Given an unsorted array of integers, find the length of the longest consecutive elements sequence.

For example,
Given [100, 4, 200, 1, 3, 2],
The longest consecutive elements sequence is [1, 2, 3, 4]. Return its length: 4.

Your algorithm should run in O(n) complexity.

思路：

因为要不断地确认 一个数的+1、-1是否在数组中，如果遍历数组来查找显然时间复杂度要大于O(n)，考虑用HashSet可以方便的知道一个元素是否在集合中，如果一个元素+1/-1都不在集合中，则删去该元素，并且每一个元素判断后都删除，则时间复杂度为O(n)。
算法：

 public int longestConsecutive(int[] nums) {
     int max = 1;
     Set<Integer> set = new HashSet<Integer>();
     for (int i = 0; i < nums.length; i++) {
         set.add(nums[i]);
     }
     for (int i = 0; i < nums.length; i++) {
         int length = 1; // 当前元素连续长度
         if (set.contains(nums[i] + 1) || set.contains(nums[i] - 1)) {
             int cur = nums[i];
             while (set.contains(cur + 1)) {
                 length++;
                 set.remove(cur + 1);
                 cur++;
             }
             cur = nums[i];
             while (set.contains(cur - 1)) {
                 length++;
                 set.remove(cur - 1);
                 cur--;
             }
         }
         set.remove(nums[i]);
         max = Math.max(length, max);
     }
     return max;
 }