[LeetCode] 128. Longest Consecutive Sequence

题：https://leetcode.com/problems/longest-consecutive-sequence/description/

题目

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

Your algorithm should run in O(n) complexity.

Example:

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

思路

两种方式

1.

利用sort先对nums进行排序，然后遍历一次，如果当前遍历的和上次访问的数据相同则跳过，否则，若当前数据比上次访问的数据大一，则不同。

2.

利用 set对nums进行set操作。

Solution

https://leetcode.com/problems/longest-consecutive-sequence/solution/

code

1.

class Solution:
    def longestConsecutive(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        if len(nums) == 0:
            return 0
        nums.sort()

        longest_streak = 1
        current_streak = 1

        for i in range(1,len(nums)):
            if nums[i]!=nums[i-1]:
                if nums[i] == nums[i-1]+1:
                    current_streak +=1
                else:
                    longest_streak = max(longest_streak,current_streak)
                    current_streak =1
        return max(longest_streak,current_streak)

2.

class Solution:
    def longestConsecutive(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        longest_streak = 0
        numsset = set(nums)

        for num in nums:
            if num-1 not in numsset:
                current_num = num
                current_streak = 1
                while current_num + 1 in numsset:
                    current_num +=1
                    current_streak +=1
                longest_streak = max(longest_streak,current_streak)

        return longest_streak