LeetCode – Longest Consecutive Sequence (Java)

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 should be [1, 2, 3, 4]. Its length is 4.

Your algorithm should run in O(n) complexity.


Because it requires O(n) complexity, we can not solve the problem by sorting the array first. Sorting takes at least O(nlogn) time.

Java Solution

We can use a HashSet to add and remove elements. HashSet is implemented by using a hash table. Elements are not ordered. The add, remove and contains methods have constant time complexity O(1).

public static int longestConsecutive(int[] num) {
	// if array is empty, return 0
	if (num.length == 0) {
		return 0;
	Set<Integer> set = new HashSet<Integer>();
	int max = 1;
	for (int e : num)
	for (int e : num) {
		int left = e - 1;
		int right = e + 1;
		int count = 1;
		while (set.contains(left)) {
		while (set.contains(right)) {
		max = Math.max(count, max);
	return max;

After an element is checked, it should be removed from the set. Otherwise, time complexity would be O(mn) in which m is the average length of all consecutive sequences.

To clearly see the time complexity, I suggest you use some simple examples and manually execute the program. For example, given an array {1,2,4,5,3}, the program time is m. m is the length of longest consecutive sequence.

Category >> Algorithms  
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  • Vaibhav Gupta

    for input {8,7,6,5,4,3,2,1} the complexity of this solution will be O(n^2). The second for loop, when run with e=8 will remove all the elements from the set. Thus, for e=8, the while loop that runs, will be of O(n) and after that, the for loop will still iterator for other 7 values (though each time it will return false for set.contains(left)) but still the for loop will run n times nonetheless. The worst case complexity of this solution is thus O(n^2) and not O(n).

  • Matias SM

    Is this comment about an old implementation? in the given example m = 1 (not n). And, following the code, what would happen would be that each “set.contains” would return false, and the whole array will be iterated once more => the complexity would be O(N). What am I missing?

  • Matias SM

    TreeSet is a balanced search tree (guarantees log N cost operations). Thus, the total insertion cost is N log N (required time complexity is O(N)

  • Eugene Arnatovich

    //Longest Consecutive Sequence (Java)
    // Note: this is longest "consecutive" elements sequence, i.e., 1,2,3,4 is correct since difference
    // between neighbors is 1, e.g. 4-3=1, 3-2=1, and so
    public int solution(int[] arr){
    Set tree = new TreeSet();
    for (int a : arr) {
    Integer[] newarr = new Integer[myarr.length];
    int ans = 1;
    int max = ans;
    for (int i = 1; i ans) {
    ans = max;
    return ans;

  • Renzo Nuccitelli

    I took sometime to get in same conclusion of using a hash set. This is my solution in Python: https://github.com/renzon/code_interview_training/blob/master/longest_consecutive_sequence.py. I iterate over set itself and stop once I reach a interval its length is bigger than the number of elements on set.

  • Renzo Nuccitelli

    I had the same idea. But for huge number as max and min the running time would be terrible. Imagine [-10ˆ1000000000000000, 10ˆ1000000000000000]. Only two numbers on list and you would take a bunch of time. So it would be O(max-min). It would, but hash seem be better in this case

  • animesh

    the valid subsequences will [100,200], [4,200], [1,3], [1,2]
    All these are valid longest increasing sub sequence. But the answer can’t be 4.

  • animesh

    I don’t think this was the actual question.

    If you see the example given in leetcode,

    [10, 9, 2, 5, 3, 7, 101, 18],

    for this it takes the longest subsequence as [2, 5, 7, 101]

    It doesn’t take into account the higher numbers which came previously.

    Have you tried running the code in leetcode?

    the example you have given, [100, 4, 200, 1, 3, 2]

    when run in leetcode, gives the answer as 2 i.e., 100 and 200
    whereas you say the answer is 4.

  • coder

    I seriously doubt the correctness of this code for input array { 4,0,1,2,16,12,13,14,15,5,6,7,8,9,10};

  • dawei su

    why not remove(num) after each checking? It will not affect the result anyway. Then for the extreme case you will also get O(N).

  • Vimukthi Weerasiri

    why dont you iterate throw the set, rather than the array? It will avoid checking for the duplicate values in array.

  • ygw

    Adding “if(!set.contains(e)) continue” after the second “for” line appears to make it completely O(n).

  • Tony Wang

    If ‘set.contains(…)’ is O(1) then whole solution can be considered as O(n).
    Another solution without using of Hash is do sorting first, the average complexity will be O(NlogN), plus an O(n) to find out longest sequence.

  • PK

    post http://codeluli.blogspot.in/2013/02/longest-consecutive-sequence.html has a solution with runtime complexity O(N) :).

  • PK

    for (int e : num)

    Above code will use O(N) time right? where N is number of elements in array num. Yes add operation takes constant time O(1) to add an element in a hash set with perfect hash function, but if we repeat an operation of O(1) in a loop for N times it will have time complexity of O(N) as it time taken will grow/shrink with value of N. Am i understanding it correctly?

  • rs

    What do you mean by n == m? It doesn’t seem to me to be possible unless the whole set contains a whole consecutive sequence, in which case the runtime is still O(n). The example{1,3,5,7,9} in the article doesn’t seem to satisfy this condition?

  • Aaron Zhang

    who tells u complexity of contain is O(1)? check this http://en.wikipedia.org/wiki/Hash_table

  • Paul Sun

    In the first for loop I can get the min and max number of the array, beside put them in hash. Then I can loop through from min through max to see where longest consecutive sequence is. I think this way would be clear. If the number is very scattered, the complexity can not be much bigger than O(n).

  • ryanlr

    That is correct. Thanks for pointing it out!

  • ryanlr

    I just tested the code using {1,2,4,5,3}, the result is 5, which is correct.

  • ryanlr

    Thanks! The code is updated.

  • Mo

    Just put a check in the beginning before int max=1.

    if(num.length == 0){
    return 0;

  • cj

    wrong answer dude: think about [1 2 4 5 3]

  • Dun Liu

    if num.length==0, it will return max as 1. Which is wrong

  • violethaze

    when n = m, the complexity will be O(n^2)

  • Wellwisher