## Longest Consecutive Sequence

### 描述

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.

### 分析

### 代码

{% if book.java %}
```java
// Longest Consecutive Sequence
// Time Complexity: O(n)，Space Complexity: O(n)
public class Solution {
public int longestConsecutive(int[] nums) {
final HashSet mySet = new HashSet();
for (int i : nums) mySet.add(i);

int longest = 0;
for (int i : nums) {
int length = 1;
for (int j = i - 1; mySet.contains(j); --j) {
mySet.remove(j);
++length;
}
for (int j = i + 1; mySet.contains(j); ++j) {
mySet.remove(j);
++length;
}
longest = Math.max(longest, length);
}
return longest;
}
}
```
{% endif %}

{% if book.cpp %}
```cpp
// Longest Consecutive Sequence
// Time Complexity: O(n)，Space Complexity: O(n)
class Solution {
public:
int longestConsecutive(const vector &nums) {
unordered_set my_set;
for (auto i : nums) my_set.insert(i);

int longest = 0;
for (auto i : nums) {
int length = 1;
for (int j = i - 1; my_set.find(j) != my_set.end(); --j) {
my_set.erase(j);
++length;
}
for (int j = i + 1; my_set.find(j) != my_set.end(); ++j) {
my_set.erase(j);
++length;
}
longest = max(longest, length);
}
return longest;
}
};
```
{% endif %}