動機

令人回想到73 如果找不到範圍就先看看怎麼定義頭吧

Problem

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: 4Explanation: 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 <= 105
  • -109 <= nums[i] <= 109

Sol

用set去存所有數字不難，但是怎麼找出sequence就會頭痛了

會頭痛是因為不知道怎麼找出sequence的範圍，如果列舉每個數字去找最大長度，會有許多重複計算

如果是sequence會有頭與尾

那要找頭還是尾? 其實都可以，下面是找頭

這讓我想起73把第一行與列當成mark sheet來用

class Solution:
    def longestConsecutive(self, nums):
        nums = set(nums)

        ret = 0
        for n in nums:
            if n-1 not in nums:
                tmp = 1
                while n+1 in nums:
                    n,tmp = n+1, tmp+1
                ret = max(ret,tmp)
        return ret