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[Leetcode] Longest Consecutive Sequence
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:
排序的话至少要O(nlgn) 的复杂度。O(n)的复杂度，目前只找到了使用hash来解决的方案，add, remove, contains 等方法的复杂度都是 O(1)，因此两次遍历的操作复杂度为 O(n)。
public class Solution { public int longestConsecutive(int[] num) { if(num==null||num.length==0) return 0; HashSet<Integer> hs=new HashSet<Integer>(); for(int i:num){ hs.add(i); } int max=1; for(int e:num){ int left=e-1; int right=e+1; int temp=1; while(hs.contains(left)){ hs.remove(left); //其实就相当于把num数组给group掉，每个元素只被走了一遍 temp++; left--; } while(hs.contains(right)){ hs.remove(right); //同上 right++; temp++; } max=Math.max(max, temp); } return max; }}
[Leetcode] Longest Consecutive Sequence
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