0128. Longest Consecutive Sequence

---

Given an unsorted array of integers nums, return the length of the longest consecutive elements sequence.

You must write an algorithm that runs in O(n) time.

Example 1:

Input: nums = [100,4,200,1,3,2]
Output: 4
Explanation: The longest consecutive elements sequence is [1, 2, 3, 4]. Therefore its length is 4.

Example 2:

Input: nums = [0,3,7,2,5,8,4,6,0,1]
Output: 9

Constraints:

• 0 <= nums.length <= 10^5
• -109 <= nums[i] <= 10^9

---

解法1

key是怎么重复利用之间的计算结果

class Solution {
    public int longestConsecutive(int[] nums) {
        int res = 0;
        Map<Integer, Integer> memo = new HashMap<Integer, Integer>();
        for(int n : nums){
            if(memo.containsKey(n))
                continue;
            
            int left = memo.getOrDefault(n - 1, 0);
            int right = memo.getOrDefault(n + 1, 0);
            // 当前长度等于左链＋右链＋１
            int sum = left + right + 1;
            
            // 更新最大值
            res = Math.max(res, sum);
            
            // 计算结果放入memo中
            if(left > 0)
                memo.put(n - left, sum);
            if(right > 0)
                memo.put(n + right, sum);
            memo.put(n, sum);
        }
        
        return res;
    }
}