Longest Consecutive Sequence

题目描述：

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.
解题思路：
首先使用一个hashtable保存输入数组nums中的值，hashtable的key值为数组中的元素，value值为对应数组元素在数组中的位置；

然后对于数组nums中每一个元素nums[i]，遍历从该元素开始递增和递减的元素是否在数组nums中；由于使用hashtable保存数组nums

中的元素，所以可以在O(1)时间判断一个元素是否在数组nums中，如果递增或递减得到的元素在nums中，那么连续序列的长度加1，

并把以该元素为key值的键值对从hashtable中删除。

时间复杂度O(n)

AC代码如下：

class Solution{
public:
int longestConsecutive(vector<int> &nums)
{
unordered_map<int, int> hashtable;
for (int i = 0; i < nums.size(); ++i){
hashtable[nums[i]] = i;
}
int count = 0;
int max = 0;
for (int i = 0; i < nums.size(); ++i){
count = 1;
int curIncrease = nums[i] + 1;
while (hashtable.find(curIncrease) != hashtable.end()){
++count;
hashtable.erase(hashtable.find(curIncrease));
++curIncrease;
}
int curDecrease = nums[i] - 1;
while (hashtable.find(curDecrease) != hashtable.end()){
++count;
hashtable.erase(hashtable.find(curDecrease));
--curDecrease;
}
if (count>max) max = count;
}
return max;
}
};