LeetCode OJ 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.

class Solution {public:    int longestConsecutive(vector<int> &num) {        map<int, int> m;        for (int i = 0; i < num.size(); i++) {                        if (m.count(num[i])) continue;            m[num[i]] = num[i];            if (m.count(num[i] - 1)) {                m[num[i]] = m[num[i] - 1];                m[m[num[i]]] = num[i];            }            if (m.count(num[i] + 1)) {                                int tail = m[num[i] + 1];                m[tail] = m[num[i]];                m[m[num[i]]] = tail;            }        }        map<int, int>::iterator iter;        int Max = 0;        for (iter = m.begin(); iter != m.end(); iter++) {            if (ABS(iter->first, iter->second) > Max) Max = ABS(iter->first, iter->second);        }        return Max + 1;    }    int ABS(int a, int b) {        return a - b < 0 ? b - a : a - b;    }};