Longest Consecutive Sequence

Desicription

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

Your algorithm should run in O(n) complexity.

Example:

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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.

Solution

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class Solution {
public:
int longestConsecutive(vector<int>& nums) {
unordered_set<int> unSet(nums.begin(), nums.end());
int res = 0;
for(int num : nums) {
if(unSet.find(num) == unSet.end())
continue;
int pre = num - 1;
int next = num + 1;
while(unSet.find(pre) != unSet.end())
unSet.erase(pre--);
while(unSet.find(next) != unSet.end())
unSet.erase(next++);
res = max(res, next - pre - 1);
}
return res;
}
};