LeetCode-128.Longest Consecutive Sequence

https://leetcode.com/problems/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.

首先将输入nums存放在set中便于查找。将nums元素转化为图的节点，对于每一个节点，在set中寻找其相邻元素，如果找到，则长度+1，并将其从set中移除

int longestConsecutive(vector<int>& nums)
{
unordered_set<int> set(nums.begin(), nums.end());
int len,n_copy,res=1;
for (int n:nums)
{
if (set.find(n) != set.end())
{
set.erase(n);
len = 1;
n_copy = n;
while (set.find(++n_copy) != set.end())
{
set.erase(n_copy);
len++;
}

n_copy = n;
while (set.find(--n_copy) != set.end())
{
set.erase(n_copy);
len++;
}
res = max(res, len);
}
}
return res;
}