LeetCode 136. Single Number



Given a non-empty array of integers, every element appears twice except for one. Find that single one.


Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

Example 1:

Input: [2,2,1]
Output: 1

Example 2:

Input: [4,1,2,1,2]
Output: 4


Use hash table to searching the elements that occur only once.

Python Solution

class Solution:
    def singleNumber(self, nums: List[int]) -> int:
        count = {}
        for num in nums:
            if num in count:
                count[num] += 1
                count[num] = 1
        for key, value in count.items():
            if value == 1:
                return key
  • Time complexity: O(N).
  • Space complexity: O(N).

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