128 最长连续序列

https://leetcode.cn/leetbook/read/top-interview-questions-hard/xwgcuv/

cost: 19m4s

class Solution {
public:
    int longestConsecutive(vector<int>& nums) {
        unordered_set<int> numSet;
        for (const int num : nums) {
            numSet.insert(num);
        }
        int res = 0;
        unordered_set<int> visited;
        for (auto key : numSet) {
            int lower = key - 1;
            while (numSet.count(lower) && !visited.count(lower)) {
                visited.insert(lower);
                lower--;
            }
            int upper = key + 1;
            while (numSet.count(upper) && !visited.count(upper)) {
                visited.insert(upper);
                upper++;
            }
            res = std::max(res, upper - lower - 1);
        }
        return res;
    }
};