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.

    map<int, int> mp;

    int longestConsecutive(vector<int> &num) 
    {
        int i;
        // map all the numbers and set value to 1 (not accessed state)
        for (i = 0; i < num.size(); i++)
        {
            mp[num[i]] = 1;
        }

        int res = 0;
        for (i = 0; i<num.size(); i++){
            int sum = 1;
            if (mp.count(num[i]))
            {
                mp[num[i]] = 0;
                int left = num[i] - 1;
                // if the left number exist in the map and not accessed
                while (mp.count(left) && mp[left] != 0)
                {
                    mp[left--] = 0;
                    sum++;
                }
                int right = num[i] + 1;
                // if the right member exist in the map and not accessed
                while (mp.count(right) && mp[right] != 0)
                {
                    mp[right++] = 0;
                    sum++;
                }
            }
            if (res<sum)
                res = sum;
        }
        return res;
    };

int _tmain(int argc, _TCHAR* argv[])
{
    Solution *sln = new Solution();
vector<int> svec;
    svec.push_back(100);
    svec.push_back(4);
    svec.push_back(200);
    svec.push_back(3);
    svec.push_back(101);
    svec.push_back(2);
    svec.push_back(3);
    svec.push_back(99);
    svec.push_back(2);
    svec.push_back(98);
    svec.push_back(96);
    svec.push_back(97);

    int ret = sln->longestConsecutive(svec);
    cout << ret << endl;

    return 0;
}