Leetcode128——Longest Consecutive Sequence

文章作者：Tyan
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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 map = new HashMap();
        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x
            x = 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;
    }
}