# 128. Longest Consecutive Sequence

Given an unsorted array of integers, find the length of the longest consecutive elements sequence.

Your algorithm should run in O(*n*) complexity.

**Example:**

```
Input: [100, 4, 200, 1, 3, 2]
Output: 4
Explanation: The longest consecutive elements sequence is [1, 2, 3, 4]. Therefore its length is 4.
```

```cpp
// Hashset
int longestConsecutive(vector<int>& nums) { // time: O(n); space: O(n)
    unordered_set<int> st(nums.begin(), nums.end());
    int res = 0;
    for (int num : nums) {
        if (!st.count(num)) continue;
        st.erase(num);
        int prev = num - 1, next = num + 1;
        while (st.count(prev)) {
            st.erase(prev--);
        }
        while (st.count(next)) {
            st.erase(next++);
        }
        res = max(res, next - prev - 1);
    }
    return res;
}
```

```cpp
// Hashmap
int longestConsecutive(vector<int>& nums) { // time: O(n); space: O(n)
    unordered_map<int, int> m;
    int res = 0;
    for (int num : nums) {
        if (m.count(num)) continue;
        int left = m.count(num - 1) ? m[num - 1] : 0;
        int right = m.count(num + 1) ? m[num + 1] : 0;
        int sum = left + 1 + right;
        m[num] = sum;
        res = max(res, sum);
        m[num - left] = sum;
        m[num + right] = sum;
    }
    return res;
}
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://jimmylin1991.gitbook.io/practice-of-algorithm-problems/array/128.-longest-consecutive-sequence.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.