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.

// https://oj.leetcode.com/problems/longest-consecutive-sequence/ // Author : Chao Zeng
// Date   : 2015-3-30
class Solution {
public:
    int longestConsecutive(vector<int> &num) {
        map<int,int> mp;
        const int NUM = 100;//由于数据中存在负数，所以数组元素加一个偏移量，保证数组下标有效性
        int length = num.size();
        for (int i = 0; i < length; i++){
            mp[num[i] + NUM] = 1;
        }

        int ans;
        int res = 0;
        for (int i = 0; i < length; i++){
            ans = 1;
            if (mp.count(num[i] + NUM)){
                int left = num[i] + NUM - 1;
                while (mp.count(left) && mp[left] != 0){
                    mp[left--] = 0;
                    ans++;
                }

                int right = num[i] + NUM + 1;
                while (mp.count(right) && mp[right] != 0){
                    mp[right++] = 0;
                    ans++;
                }
            }

            if (res < ans){
                res = ans;
            }
        }
        return res;
    }
};