给定无序数组，求最大的连续的数的长度

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.

class Solution {
public:
    int longestConsecutive(vector<int> &num) {
        map<int, bool> flagMap;
        int nMaxLen = 1;

        for (vector<int>::iterator itr=num.begin(); itr!=num.end();++itr)
            flagMap[*itr] = true;

        for (vector<int>::iterator itr=num.begin(); itr!=num.end();++itr)
        {
            int nCount = 1;
            if (flagMap[*itr])
            {
                flagMap[*itr] = false;      // mark false

                int nLeft = *itr-1;
                while (flagMap.find(nLeft)!=flagMap.end() && flagMap[nLeft]){
                    flagMap[nLeft] = false;
                    ++nCount;
                    nLeft -= 1;
                }

                int nRight = *itr + 1;
                while (flagMap.find(nRight)!=flagMap.end() && flagMap[nRight]){
                    flagMap[nRight] = false;
                    ++nCount;
                    nRight += 1;
                }
            }

            if (nCount > nMaxLen)
            nMaxLen = nCount;
        }

        return nMaxLen;
    }

};