Description
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.
Solution
HashSet, time O(n), space O(n)
将nums存在HashSet中，然后遍历nums，对于每个元素都尽量向两边扩展，同时移除set中元素，就行了。
class Solution {
public int longestConsecutive(int[] nums) {
if (nums == null) {
return -1;
}
Set<Integer> set = new HashSet<>();
for (int n : nums) {
set.add(n);
}
int maxLen = 0;
for (int n : nums) {
if (!set.contains(n)) { // already used
continue;
}
int len = 1;
set.remove(n);
// extend left
for (int i = n - 1; set.contains(i); --i) {
++len;
set.remove(i);
}
// extend right
for (int i = n + 1; set.contains(i); ++i) {
++len;
set.remove(i);
}
maxLen = Math.max(maxLen, len);
}
return maxLen;
}
}
