LeetCode题解：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.

题意：给定未排序的整数数组，找出最长连续区间的长度。要求时间复杂度为O(n)

思路：用一个哈希表保存数组每一个元素作为区间终点时可达到的最大长度，在添加元素到哈希表的过程中可能会更新表内数据

代码

public class Solution {
    public int longestConsecutive(int[] num) {
        int res = 0;
        HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
        for (int n : num) {
            if (!map.containsKey(n)) {
                int left = (map.containsKey(n - 1)) ? map.get(n - 1) : 0;
                int right = (map.containsKey(n + 1)) ? map.get(n + 1) : 0;
                // sum: length of the sequence n is in
                int sum = left + right + 1;
                map.put(n, sum);

                res = Math.max(res, sum);

                map.put(n - left, sum);
                map.put(n + right, sum);
            }
            else {
                continue;
            }
        }
        return res;
    }
}