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：给出一个无序数组，求这个无序数组中最大的连续数组长度，要求时间复杂度是O(n)。
思路：自己没有想到解题思路，参考的是http://www.cnblogs.com/grandyang/p/4276225.html给出的解法。用set标记全部元素，然后遍历数组，找每个元素的最左和最右，找的部分都从set中移除，然后求左右边界差值，更新最大长度。
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;
}