128. Longest Consecutive Sequence

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.

题意及分析：一个数组中连续的元素序列的长度。

public int longestConsecutive(int[] nums) {
  Set<Integer> set = new HashSet<>();
  for (int num : nums) {
    set.add(num);       //保持nums[i]中数的唯一性
  }
  int max = 0;
  for (int i = 0; i < nums.length; i++) {
    if (set.contains(nums[i])) {
      int count = 1;

      int low = nums[i] - 1; // 看下set中是否包含比当前元素小一的元素
      while (set.contains(low)) {
        count++;
        low--;
      }

      int high = nums[i] + 1;// 看下set中是否包含比当前元素小一的元素
      while (set.contains(high)) {
        count++;
        high++;
      }
      max = Math.max(max, count);
    }
  }
  return max;
}