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.

　　The key point is that for each number say Val in the array, we only need to determine whether its neighbor (Val-1 and Val+1) is in the array and  for a consecutive sequence we only need to know the the boundary and the length information。

class Solution {

public:

    int longestConsecutive(vector<int> &num) {

        // Start typing your C/C++ solution below

        // DO NOT write int main() function

        int size = num.size();

        if(size < 2) return size;

        

        map<int, int> myMap;

        for(int i = 0; i< num.size(); i++)

             myMap.insert(pair<int, int>(num[i],1));

        

        int maxLen = 0;

        map<int, int>::iterator m_it;

        map<int, int>::iterator temp_it;

        

        for(m_it = myMap.begin(); m_it != myMap.end();m_it++)

        {

        

            if(m_it->second == 0) continue;

            int len = 1;

            int low = m_it->first;

            int high = low;

            while(true)

            {

              temp_it= myMap.find(low-1);

              if(temp_it == myMap.end())

                break;

              low--;

              len++;

              temp_it->second = 0;

            }

            

            while(true)

            {

                 temp_it=   myMap.find(high+1) ;

                 if(temp_it == myMap.end())

                break;

                high++;

                len++ ;

                temp_it->second = 0;

            }

            

            maxLen = maxLen < len ? len : maxLen ; 

            

            

        }

        

        

        return maxLen ;

        

        

    }

};