Longest Consecutive Sequence

Q:

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.

T:
First store all numbers in a hashmap. Then traverse the array, for each number in the map, see if its previous numbers or next numbers also in the map.

A:

class Solution {
public:
    int longestConsecutive(vector &num) {
        unordered_map< int, bool > mark;
        for(int i=0; i< num.size(); i++) {
            mark[num[i]] = true;
        }
        int maxlen = 0;
        for(int i=0; i< num.size(); i++) {
            mark[num[i]] = false;
            int mx = 1;
            int v = num[i] + 1;
            while(mark[v]) {
                mark[v] = false;
                mx++;
                v++;
            }
            v = num[i] - 1;
            while(mark[v]) {
                mark[v] = false;
                mx++;
                v--;
            }
            if(mx > maxlen) {
                maxlen = mx;
            }
        }
        return maxlen;
    }
};