LeetCode：Longest Consecutive Sequence

Problem:

Longest Consecutive Sequence

 Total Accepted: 3391 Total Submissions: 13151My Submissions

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.

用一个哈希表记录每个元素是否用过，另外，对每个元素，以该元素为中心向两边扩张即可方便地找出最长的长度。

class Solution {public:       int longestConsecutive(vector<int> const& num)    {   hash_map<int,bool>hashlist;   for (int i = 0; i < num.size(); i++)   {   hashlist[num[i]]=true;   }   int longest=0;   for (int i = 0; i < num.size(); i++)   {   if (hashlist[num[i]]==false)   {   continue;   }   int length=1;   int k1=num[i];int k2=k1;   while(hashlist[k1+1])   {   hashlist[k1+1]=false;   length++;   k1++;   }      while(hashlist[k2-1]&&k2-1>=0)   {   hashlist[k2-1]=false;   length++;   k2--;   }   longest=max(longest,length);        }   return longest;   }};