leetcode[128]Longest Consecutive Sequence

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.

class Solution {
public:
int longestConsecutive(vector<int> &num) {
if(num.empty())return 0;
unordered_set<int> all;
unordered_set<int> visited;
for(int i=0;i<num.size();i++)
all.insert(num[i]);
int max=0;
for(int i=0;i<num.size();i++)
{
if(visited.find(num[i])!=visited.end())continue;
int count=1;
visited.insert(num[i]);
int left=num[i];
while(all.find(--left)!=all.end())
{
count++;
visited.insert(left);
}
int right=num[i];
while(all.find(++right)!=all.end())
{
count++;
visited.insert(right);
}
max=max>count?max:count;
}
return max;
}
};