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.

这道题一开始想先排序，然后找，但是时间复杂度超过O(n). 参考了网上的资料，思路如下：

1.将原数组存到hash_set里面。因为查找时间复杂度O(1).

2.遍历数组，每次在hash表里面找它的+1和-1，找到连续的数，则往该方向（+1或者-1方向）继续找连续数。

3.统计+方向和-方向的连续数的个数即为所求。

int getNum(set<int> &map, int cur, bool up){    int num = 0;    while(map.find(cur) != map.end())    {        map.erase(cur);        num++;        up?cur++:cur--;    }    return num;}int longestConsecutive(vector<int> &num){    set<int> map;    for(int i = 0; i < num.size(); ++i)        map.insert(num[i]);    int max = 1;    for(int i = 0; i< num.size(); ++i)    {        if(map.find(num[i]) != map.end())        {            int temp = 1;            int cur = num[i];            map.erase(cur);            temp += getNum(map, cur + 1, true);            temp += getNum(map, cur - 1, false);            max = temp > max ? temp : max;        }    }    return max;}