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.

分析：主要用map来记录nums[i]是否检查过。检查过则从map中删除，同时检查nums[i] + 1和nums[i] - 1。

运行时间：40ms

class Solution {
public:
    int longestConsecutive(vector<int>& nums) {
        map<int, int> hashmap;
        for(int i = 0; i < nums.size(); i++) hashmap.insert(map<int, int>::value_type(nums[i], i));
        
        int longest = 1;
        map<int, int>::iterator ite;
        for(ite = hashmap.begin(); ite != hashmap.end(); ite++){
            int len = 1;
            
            int j = ite->first + 1;
            for(; hashmap.find(j) != hashmap.end(); j++){
                len++;
                hashmap.erase(j);
            }
            int k = ite->first - 1;
            for(; hashmap.find(k) != hashmap.end(); k--){
                len++;
                hashmap.erase(k);
            }
            longest = max(longest, len);
        }
        return longest;
    }
};