Tech Issues – Zhijun Chen https://zhijunchen.com Fri, 07 Jun 2024 06:32:12 +0000 en-US hourly 1 https://wordpress.org/?v=6.5.4 https://zhijunchen.com/wp-content/uploads/2021/03/cropped-my-site-icon-1-32x32.png Tech Issues – Zhijun Chen https://zhijunchen.com 32 32 Find Single Number In An Array https://zhijunchen.com/find-single-number-in-an-array/ Thu, 29 Apr 2021 10:25:23 +0000 https://zhijunchen.com/?p=149 I came across the following array questions on LeetCode and was amazed by one of the solutions achieving a linear runtime complexity and without using extra memory. Declaration: I didn’t come up with the solution.

Problem Statement

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

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 * 104
  • -3 * 104 <= nums[i] <= 3 * 104
  • Each element in the array appears twice except for one element which appears only once.

Solution: Bit Manipulation

Concept

  • If we take XOR of zero and some bit, it will return that bit
    • a ⊕ 0 = a
  • If we take XOR of two same bits, it will return 0
    • 0 ⊕ a = 0
  • a ⊕ b ⊕ a = (a ⊕ a) ⊕ b = 0 ⊕ b = b

So we can XOR all bits together to find the unique number. Here is the solution for Java: 

class Solution {
  public int singleNumber(int[] nums) {
    int a = 0;
    for (int i : nums) {
      a ^= i;
    }
    return a;
  }
}

Complexity Analysis

  • Time complexity : O(n). We only iterate through nums, so the time complexity is the number of elements in nums.
  • Space complexity : O(1).
]]> Fix Signature Invalid Issue from packages.sury.org https://zhijunchen.com/fix-signature-invalid-issue-from-packages-sury-org/ Thu, 01 Apr 2021 09:23:01 +0000 https://zhijunchen.com/?p=71 I came across the following error when trying to update Debian packages.

*********@*******:~$ sudo apt-get update
...
Err:11 https://packages.sury.org/php stretch InRelease
  The following signatures were invalid: EXPKEYSIG B188E2B695BD4743 DEB.SURY.ORG Automatic Signing Key <deb@sury.org>
Get:15 https://packages.cloud.google.com/apt cloud-sdk/main amd64 Packages [154 kB]
Fetched 161 kB in 1s (134 kB/s)   
Reading package lists... Done
W: An error occurred during the signature verification. The repository is not updated and the previous index files 
will be used. GPG error: https://packages.sury.org/php stretch InRelease: The following signatures were invalid: EX
PKEYSIG B188E2B695BD4743 DEB.SURY.ORG Automatic Signing Key <deb@sury.org>
W: Failed to fetch https://packages.sury.org/php/dists/stretch/InRelease  The following signatures were invalid: EX
PKEYSIG B188E2B695BD4743 DEB.SURY.ORG Automatic Signing Key <deb@sury.org>
W: Some index files failed to download. They have been ignored, or old ones used instead.

It turned out to be a known expired key issue occurred recently. The following commands should fix it.

sudo apt-key del B188E2B695BD4743
sudo wget -O /etc/apt/trusted.gpg.d/php.gpg https://packages.sury.org/php/apt.gpg
]]>