Longest Consecutive Sequence
阿新 • • 發佈:2017-11-10
best num clas while algorithm 整數 連續序列 problem its
Nov, 9, 2017.
https://leetcode.com/problemset/algorithms/?topicSlugs=union-find%2Clinked-list
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.
建立字典,依次讀入整數,每次在邊界處記錄長度變化,為防止重復計算,整數必須被讀入字典。
復雜度為O(N),以下是submission。
1 class Solution: 2 def longestConsecutive(self, nums): 3 length = {} 4 ans = 0 5 for i in nums: 6 if i not in length: 7 l = length[i - 1] if i - 1 in length else 08 r = length[i + 1] if i + 1 in length else 0 9 length[i] = length[i - l] = length[i + r] = l + r + 1 10 ans = max(ans, length[i]) 11 return ans
解法二,數組->哈希表,尋找連續序列起始數(x-1 not in nums),直接查找這個序列的長度。
有趣的地方在於,我們是在哈希表裏查詢一個確定的數,使得這個操作的復雜度被壓進了O(1)。因此這個算法的復雜度還是O(N)。
以下是StefanPochmann的submission。
1 def longestConsecutive(self, nums): 2 nums = set(nums) 3 best = 0 4 for x in nums: 5 if x - 1 not in nums: 6 y = x + 1 7 while y in nums: 8 y += 1 9 best = max(best, y - x) 10 return best
其實兩種方法都利用了哈希表查詢的便利性。我原來想把輸入看作森林,用並查集來完成工作。這種算法不好實現,也不見得簡單。
Longest Consecutive Sequence