# 128 Longest Consecutive Sequence

### 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.

### Solutions:

``````public class Solution {
public int longestConsecutive(int[] nums) {
HashMap<Integer, Integer> left = new HashMap<Integer, Integer>();
HashMap<Integer, Integer> right = new HashMap<Integer, Integer>();
HashSet<Integer> visited = new HashSet<Integer>();
int max = 0;
for (int i = 0; i < nums.length; i ++) {
if (visited.contains(nums[i])) {
continue;
}
if (!right.containsKey(nums[i] - 1) && !left.containsKey(nums[i] + 1)) {
left.put(nums[i], nums[i]);
right.put(nums[i], nums[i]);
max = Math.max(max, 1);
continue;
}
if (right.containsKey(nums[i] - 1) && left.containsKey(nums[i] + 1)) {
int a = right.get(nums[i] - 1);
int b = left.get(nums[i] + 1);
right.remove(nums[i] - 1);
left.remove(nums[i] + 1);
left.put(a, b);
right.put(b, a);
max = Math.max(max, b - a + 1);
continue;
}
if (right.containsKey(nums[i] - 1)) {
int a = right.get(nums[i] - 1);
int b = nums[i] - 1;
right.remove(b);
left.put(a, nums[i]);
right.put(nums[i], a);
max = Math.max(max, b - a + 2);
}
if (left.containsKey(nums[i] + 1)) {
int a = nums[i] + 1;
int b = left.get(nums[i] + 1);
left.remove(a);
left.put(nums[i], b);
right.put(b, nums[i]);
max = Math.max(max, b - a + 2);
}
}
return max;
}
}
``````
``````public class Solution {
public int longestConsecutive(int[] nums) {
HashSet<Integer> data = new HashSet<Integer>();
for (int i = 0; i < nums.length; i ++) {
}
int max = 0;
for (int i = 0; i < nums.length; i ++) {
if (data.contains(nums[i])) {
int count = 1;
int tmp = nums[i] - 1;
while (data.contains(tmp)){
data.remove(tmp);
tmp --;
count ++;
}
tmp = nums[i] + 1;
while (data.contains(tmp)) {
data.remove(tmp);
tmp ++;
count ++;
}
max = Math.max(max, count);
}
}
return max;
}
}
``````