Longest Consecutive Sequence

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. 

解决思路

1. HashMap存储元素和对应的边界长度；

2. 插入元素n时，检查n-1和n+1是否在map中，如果在，则更新边界的长度，最后输出最大长度即可。

程序

public class Solution {
    public int longestConsecutive(int[] nums) {
        if (nums == null || nums.length == 0) {
            return 0;
        }
        
        HashMap<Integer, Integer> map = new HashMap<>();
        int max = 1;
        
        for (int n : nums) {
            if (map.containsKey(n)) {
                continue;
            }
            map.put(n, 1);
            if (map.containsKey(n - 1)) {
                max = Math.max(max, updateLen(map, n - 1, n));
            }
            if (map.containsKey(n + 1)) {
                max = Math.max(max, updateLen(map, n, n + 1));
            }
        }
        
        return max;
    }
    
    private int updateLen(HashMap<Integer, Integer> map, int a, int b) {
        int lower = a - map.get(a) + 1;
        int higher = b + map.get(b) - 1;
        int len = higher - lower + 1;
        map.put(lower, len); // update bound
        map.put(higher, len);
        return len;
    }
}