Given an unsorted array of integers, find the length of the longest consecutive elements sequence.
Your algorithm should run in O(n) complexity.
Example:
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.
思路:如果不对复杂度有要求,可以考虑排序。然而此题不允许超过O(N)复杂度,所以要空间换时间。
因为题中是查找连续元素,所以快速的查找考虑哈希,而且连续元素之间值相差为1,所以据此可以在hashset中逐个元素搜索,找到最大连续子串长度。
class Solution { public int longestConsecutive(int[] nums) { Set<Integer> set = new HashSet<Integer>(); for(int i :nums){ set.add(i); } int ans = 0; for(int i : set){ if(set.contains(i-1)) continue; int now = i; int sum = 1; while(set.contains(now+1)){ now++; sum++; } ans = Math.max(ans,sum); } return ans; } }