Longest Consecutive Sequence Problem


Description

LeetCode Problem 128.

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

You must write an algorithm that runs in O(n) time.

Example 1:

1
2
3
Input: nums = [100,4,200,1,3,2]
Output: 4
Explanation: The longest consecutive elements sequence is [1, 2, 3, 4]. Therefore its length is 4.

Example 2:

1
2
Input: nums = [0,3,7,2,5,8,4,6,0,1]
Output: 9

Constraints:

  • 0 <= nums.length <= 10^5
  • -10^9 <= nums[i] <= 10^9


Sample C++ Code

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
int longestConsecutive(vector<int>& nums) {
	// inserting all elements into hashset
	unordered_set<int> s(begin(nums), end(nums)); 
	int longest = 0;
	for(auto& num : s) {
		int cur_longest = 1;
		// find consecutive elements in the backward and forward direction from num
		for(int j = 1; s.count(num - j); j++) {
			s.erase(num - j); 
			cur_longest++;
		}
		for(int j = 1; s.count(num + j); j++) {
			s.erase(num + j); 
			cur_longest++;
		}
		// update longest to hold longest consecutive sequence till now
		longest = max(longest, cur_longest);  
	}
	return longest;
}




Related Posts

Bulls And Cows Problem

LeetCode 299. You are playing the Bulls and Cows game...

Repeated DNA Sequences Problem

LeetCode 187. The DNA sequence is composed of a series...

Majority Element Problem

LeetCode 169. Given an array nums of size n, return...

Fraction To Recurring Decimal Problem

LeetCode 166. Given two integers representing the numerator and denominator...

Max Points On A Line Problem

LeetCode 149. Given an array of points where points[i] =...

Lru Cache Problem

LeetCode 146. Design a data structure that follows the constraints...