128. Longest Consecutive Sequence (HashTable)

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.

思路：O(n)时间复杂度，所以不能排序。将数据放入哈希表，这样查找时间复杂度是O(1)，遍历到某个数据，可以向前和向后找它的连续序列。再用一个哈希表存储已访问过的元素，这样保证每个元素至多被处理一次。

哈希表在C++中用unordered_set实现。set的实现是红黑树，插入查找删除的时间复杂度是O(logn)不能使用。

class Solution {
public:
    int longestConsecutive(vector<int>& nums) {
        unordered_set<int> visited;
        unordered_set<int> exist;
        int ret = 0;
        int count;
        int target;
        
        for(int i = 0; i < nums.size(); i++){
            exist.insert(nums[i]);
        }
        
        for(int i = 0; i < nums.size(); i++){
            if(visited.find(nums[i])!=visited.end()) continue;
            
            visited.insert(nums[i]);
            count = 1;
            target = nums[i];
            while(exist.find(--target)!=visited.end()){
                visited.insert(target);
                count++;
            }
            target = nums[i];
            while(exist.find(++target)!=visited.end()){
                visited.insert(target);
                count++;
            }
            if(count > ret) ret = count;
        }
        return ret;
    }
};