Data Structure(1) - Array
While preparing coding test, I realized I didn’t learn about data structure in school. So, this post and the following posts are to study about data structure including I already learn in school.
What is Array?
Array is a data structure that saves the same type of elements in the continuous memory. Array has a fixed size. When an array is defined with the size, the size will not be modified. We can access to elements in array with index and the index starts with 0.
How to Declare and Initialize an Array?
To declare an array, we need to use below format.
{type}[] {array_name};
For example, I can declare like this
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// int array
int[] IntArray;
// string array
string[] StringArray;
Now we need to initialize this array. There are a few ways to initialize it.
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// The array size is 5.
IntArray = new int[5];
StringArray = new string[5]
// Or we can declare and initialize together.
int[] intArray2 = new int[5];
string[] stringArray2 = new string[5];
// If you already know elements that you want to add into array, you can do
int[] intArray3 = new int[] {1, 2, 3, 4, 5};
// or you want do it without 'new'
int[] intArray4 = {1, 2, 3, 4, 5};
How to Access the Element in the Array?
We can access each element in the array using index.
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int element1 = intArray4[2]; // 3
// if you want to edit the element value then,
intArray4[3] = 5; // intArray = {1, 2, 3, 5, 5}
Length of Array
If you want to know the array length, you can use Length and Count().
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int len1 = intArray4.Length;
int len2 = intArray4.Count();
It is because Length is one of properties of array and Count() is LINQ Method. So Length is faster than Count().
Loop of Array
We can use for statement and foreach statement to print element.
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// using for loop
for(int i = 0; i < intArray4.Length; i++) {
Console.WriteLine(intArray4[i]);
}
// using foreach loop
foreach(int i in intArray4) {
Console.WriteLine(i);
}
Multidimensional Arrays
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// Declare and initialize
string[,] 2dArray = new string[2,2]; // 2 * 2 array
// Assign values
2dArray[0,1] = "hello"
Example Problem
Let’s solve Intersection of Two Arrays II!
Problem Description:
Given two integer arrays nums1 and nums2, return an array of their intersection. Each element in the result must appear as many times as it shows in both arrays and you may return the result in any order.
Example 1:
Input: nums1 = [1,2,2,1], nums2 = [2,2]
Output: [2,2]
Example 2:
Input: nums1 = [4,9,5], nums2 = [9,4,9,8,4]
Output: [4,9]
Explanation: [9,4] is also accepted.
Constraints:
1 <= nums1.length, nums2.length <= 1000
0 <= nums1[i], nums2[i] <= 1000
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public class Solution {
public int[] Intersect(int[] nums1, int[] nums2) {
// List Declaring - To be posted
List<int> result = new List<int>();
// Declare a new array to save nums1
int[] nums3 = nums1;
// this for loop will compare elements nums3 and nums2
foreach(int i in nums2) {
// save index of element
// if nums3 doesn't have i, then i will return -1
int index = Array.IndexOf(nums3, i);
// if nums3 include i, add the element into result list
if(index > -1) {
result.Add(nums3[index]);
// to avoid duplicate I modifed the element value
nums3[index] = -1;
}
}
// this method needs int array to return. so convert list to array.
return result.ToArray();
}
}