LeetCode 128: Longest Consecutive Sequence

Difficulty: 4

Frequency: 3

Problem:

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.
Solution:

class Solution {
public:
int longestConsecutive(vector<int> &num) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
set<int> numSet;
for(int i = 0; i<num.size(); i++)
{
numSet.insert(num[i]);
}
int answer = 0;
for(int i = 0; i<num.size()&&numSet.size()>answer; i++)
{
int i_count = 0;
if (numSet.find(num[i])!=numSet.end())
{
++i_count;
int val = num[i];
while(numSet.find(++val)!=numSet.end())
{
++i_count;
numSet.erase(val);
}

val = num[i];
while(numSet.find(--val)!=numSet.end())
{
++i_count;
numSet.erase(val);
}
answer = answer>i_count?answer:i_count;
}
}
return answer;
}
};

Notes:

This algorithm is really easy if you can think of hashmap. This is the beginning I learn how to use hashmap. Quoted from others blog:"When it concerning O(n) time with unsorted data, think about hashmap."