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.

使用一般的排序+查找算法，算法时间复杂度一定会超过o(n)，故采用hash散列表可以有效解决该问题。下面为详细代码：

class Solution {public:    int longestConsecutive(vector<int> &num) {        unordered_set<int> hashTable;        unordered_set<int> visitTable;                for(int i=0;i<num.size();i++) hashTable.insert(num[i]);        int maxlen=INT_MIN;        int curlen;                for(int i=0;i<num.size();i++)        {            if(visitTable.find(num[i])!=visitTable.end()) continue;            curlen=1;            int left=num[i]-1;            while(hashTable.find(left)!=hashTable.end())            {                curlen++;                visitTable.insert(left);                left--;            }                        int right=num[i]+1;            while(hashTable.find(right)!=hashTable.end())            {                curlen++;                visitTable.insert(right);                right++;            }                        maxlen = maxlen>curlen?maxlen:curlen;        }                return maxlen;    }};