文章作者：Tyan
博客：noahsnail.com | CSDN | 简书
1. 问题描述
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.
2. 求解
题中明确要求时间复杂度为O(n)，因此这道题肯定不能使用循环遍历。这道题主要是考察哈希表，因为哈希表每次查询的时间复杂度为O(1)。因此首先要将数组转为Map。然后分别查询每个数字的前一个数与后一个数，统计数字连续的数量。如果在哈希表中存在相邻的数，查询后应该从哈希表中删除，当然不删也可以。如果哈希表为空，则直接跳出循环，不再遍历。
public class Solution {
public int longestConsecutive(int[] nums) {int max = 0;int count = 0;Map<String, Integer> map = new HashMap<String, Integer>();for(int i = 0; i < nums.length; i++) {map.put(String.valueOf(nums[i]), nums[i]);}for(int i = 0; i < nums.length; i++) {count = 1;int x = nums[i];while(true) {int temp = --x;if(map.containsKey(String.valueOf(temp))) {map.remove(String.valueOf(temp));count++;}else {break;}}//必须重置xx = nums[i];while(true) {int temp = ++x;if(map.containsKey(String.valueOf(temp))) {map.remove(String.valueOf(temp));count++;}else {break;}}if(count > max) {max = count;}if(map.isEmpty()) {break;}}return max;}
}