128. 最长连续序列

给定一个未排序的整数数组 nums ，找出数字连续的最长序列（不要求序列元素在原数组中连续）的长度。

请你设计并实现时间复杂度为 O(n) 的算法解决此问题。

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/longest-consecutive-sequence
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维护连续区间

import java.util.HashMap;
import java.util.Map;

class Solution {
    public int longestConsecutive(int[] nums) {
        if (nums == null || nums.length == 0) {
            return 0;
        }

        int ret = 1;
        Map<Integer, Integer> startMap = new HashMap<>();
        Map<Integer, Integer> endMap = new HashMap<>();
        for (int num : nums) {
            if (startMap.containsKey(num) || endMap.containsKey(num)) {
                continue;
            }

            startMap.put(num, 1);
            endMap.put(num, 1);

            Integer next = startMap.get(num + 1);
            if (next != null) {
                ret = Math.max(ret, merge(num, num + 1, startMap, endMap));
            }
            Integer previous = endMap.get(num - 1);
            if (previous != null) {
                ret = Math.max(ret, merge(num - 1, num, startMap, endMap));
            }
        }
        return ret;
    }

    private int merge(int a, int b, Map<Integer, Integer> startMap, Map<Integer, Integer> endMap) {
        int previous = endMap.get(a);
        int next = startMap.get(b);
        int length = previous + next;
        startMap.put(a - previous + 1, length);
        endMap.remove(a);
        startMap.remove(b);
        endMap.put(b + next - 1, length);
        return length;
    }
}

哈希表遍历

class Solution {
    public int longestConsecutive(int[] nums) {
        Set<Integer> num_set = new HashSet<Integer>();
        for (int num : nums) {
            num_set.add(num);
        }

        int longestStreak = 0;

        for (int num : num_set) {
            if (!num_set.contains(num - 1)) {
                int currentNum = num;
                int currentStreak = 1;

                while (num_set.contains(currentNum + 1)) {
                    currentNum += 1;
                    currentStreak += 1;
                }

                longestStreak = Math.max(longestStreak, currentStreak);
            }
        }

        return longestStreak;
    }
}