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.

題意：給出一個(gè)無(wú)序數(shù)組，求這個(gè)無(wú)序數(shù)組中最大的連續(xù)數(shù)組長(zhǎng)度，要求時(shí)間復(fù)雜度是O(n)。

思路：自己沒(méi)有想到解題思路，參考的是http://www.cnblogs.com/grandyang/p/4276225.html給出的解法。用set標(biāo)記全部元素，然后遍歷數(shù)組，找每個(gè)元素的最左和最右，找的部分都從set中移除，然后求左右邊界差值，更新最大長(zhǎng)度。

public int longestConsecutive(int[] nums) {
    int res = 0;
    if (nums == null || nums.length == 0) {
        return res;
    }

    HashSet<Integer> set = new HashSet<>();
    for (int num : nums) {
        set.add(num);
    }
    for (int num : nums) {
        if (set.remove(num)) {
            int left = num - 1;
            while (set.remove(left)) {
                left--;
            }
            int right = num + 1;
            while (set.remove(right)) {
                right++;
            }
            res = Math.max(res, right - left - 1);
        }
    }

    return res;
}