# [leetcode] 136. Single Number _ Algorithm Problem Solve for python

## 1. Problem

136. Single Number

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

You must implement a solution with a linear runtime complexity and use only constant extra space.

Example 1:

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


Example 2:

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


Example 3:

Input: nums = [1]
Output: 1


Constraints:

• 1 <= nums.length <= 3 * 10^4
• -3 * 10^4 <= nums[i] <= 3 * 10^4
• Each element in the array appears twice except for one element which appears only once.

## 2. Solution

I solved this problem like this.

• Complexity
• Time complexity : O(N)
• Space complexity : O(N)
• Step
• Count nums using dictionary.
• Checking value is 1 and return key.
class Solution:
def singleNumber(self, nums: List[int]) -> int:
dic = {}
for num in nums:
if num not in dic:
dic[num] = 0
dic[num] += 1

for key, value in dic.items():
if value == 1:
return key

return -1


Other solution is XOR operation. That code is very short!

def singleNumber(self, nums: List[int]) -> int:
return reduce(lambda total, el: total ^ el, nums)