leetcode 128. Longest Consecutive Sequence (hard)

限制时间复杂度为o(n)，于是只能用哈希表
用一个哈希表存储所有出现过的元素，对每个元素，以该元素为中心，往左右扩张，直到不连续为止，记录下最长的长度

class Solution
{public:int longestConsecutive(vector<int> &nums){unordered_set<int> setting;int longest = 0;for (int i = 0; i < nums.size(); i++){setting.insert(nums[i]);}for (auto i : nums){int length = 1;for (int j = i - 1; setting.find(j) != setting.end(); j--){setting.erase(j);++length;}for (int j = i + 1; setting.find(j) != setting.end(); j++){setting.erase(j);++length;}longest = max(longest, length);}return longest;}
};