[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.

      First of all,if there is no time complexity,we could sort the array,and then scan the array.Unfortunately,the time complexity is O(n*log n).It's been many times,when I am asked to solve a problem in O(n) which can be solved in O(n*log n) by using classical quicksort I would use a alternative method--hash table to do it.Here is the code.

class Solution {
public:
    int longestConsecutive(vector<int> &num) {
        unordered_map<int,bool> hashmap;
        int maxlength = 0;
        int curlength = 0;
        
        for(vector<int>::iterator iter = num.begin();
            iter!=num.end();++iter){
            hashmap.insert(pair<int,bool>(*iter,true));
        }
        
        for(vector<int>::iterator iter = num.begin();
            iter!=num.end();++iter){
            int num = *iter;
            curlength = 0;
            while(hashmap.find(num) != hashmap.end()){
                ++curlength;
                hashmap.erase(num);
                ++num;
            }
            
            num = *iter-1;
            while(hashmap.find(num) != hashmap.end()){
                ++curlength;
                hashmap.erase(num);
                --num;
            }
            
            maxlength = maxlength<curlength?curlength:maxlength;
        }
        return maxlength;
    }
};