[LeetCode] 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.

我的思路

这道题在 LeetCode 上通过率也就 30% 不到，还标记为 hard 了，其实题目并不难。

使用排序，当然可以解决这个问题，但是时间复杂度为 O(nlgn) ，显然不符合我们的题意。这道题我们需要考虑使用 hashmap ，以下是我 AC 后的代码，注意 LeetCode 支持c++11。

/**

 * Zhou J

 */



class Solution 

{

public:

    int longestConsecutive(vector<int> &num) 

    {

        unordered_map<int, int> hs;

        for (const auto &ival : num)

        {

            hs.insert(make_pair(ival, 0));

        }

        

        int maxl = 1;

        

        for (const auto &ival : num)

        {

            if (hs.find(ival)->second == 1)

            {

                continue;

            }

            

            // initialize the current_max before every loop

            int current_max = 1;

            

            // record the right side

            int tmp = ival;

            while(hs.count(tmp + 1))

            {

                current_max ++;

                tmp ++;

                hs[tmp] = 1;

            }

            

            // record the left side

            tmp = ival;

            while(hs.count(tmp - 1))

            {

                current_max ++;

                tmp --;

                hs[tmp] = 1;

            }

            

            // update the returned maxl

            maxl = max(current_max, maxl);

        }

        

        return maxl;

    }

};