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.

由于时间复杂度要求为O(n)，所以只能使用容器map的find和erase。否则可以排序之后按顺序查找，但是那样的时间复杂度为O(n2)。

代码如下：

class Solution {public:    int longestConsecutive(vector<int> &num) {        map<int, int> hmap;        hmap.clear();        int n = num.size();        for(int i=0; i<n; i++)        {            hmap.insert(pair<int, int>(num[i], i));        }        int result=0, cnt=0;        map<int, int>::iterator it;        for(int i=0; i<num.size(); i++)        {            int cur = num[i];            it = hmap.find(num[i]);            cnt++;            if(it!=hmap.end()){                map<int, int>::iterator iter;                while(1)                {                    iter = hmap.find(++cur);                    if(iter==hmap.end())                        break;                    cnt++;                       hmap.erase(iter);                }                cur=num[i];                while(1)                {                    iter = hmap.find(--cur);                    if(iter==hmap.end())                        break;                    cnt++;                        hmap.erase(iter);                }                if(result<cnt)                    result = cnt;                           cnt=0;            }            cnt=0;        }        return result;    }};