Hello, everyone! In this post, we will solve an algorithm problem that helps prepare for resigning. Algorithm problems are one of the commonly asked types in coding tests and are essential for skill building. We will focus on solving problems using C#.
Problem: Find Two Sum in an Array
Problem Description: Given an integer array and a specific integer (target), find the indices of the two numbers in the array that sum up to the target. It is assumed that there is exactly one solution for each input, and you may not use the same element twice. The result should be returned as an array of indices, sorted in ascending order.
Example:
Input: nums = [2, 7, 11, 15], target = 9
Output: [0, 1] // nums[0] + nums[1] = 2 + 7 = 9
Solution Process:
This problem can be approached in various ways. However, the most efficient method is to use a hashmap. By using a hashmap, we can solve it with a time complexity of O(n). Below is a step-by-step explanation of the solution process.
Step 1: Understand the Problem
First, we need to find the case where the sum of two numbers in the given array equals the target. Since we need to return the indices, we also have to remember the index of each number while calculating the sum.
Step 2: Set Up Hashmap Structure
In C#, we can implement a hashmap using Dictionary. This structure allows us to quickly find values without allowing duplicate numbers.
Step 3: Check Using a Loop
Iterate through the array one by one, finding the value that corresponds to the current number subtracted from the target. If this value exists in the hashmap, we return the indices of the two numbers.
Step 4: Implement the Code
using System;
using System.Collections.Generic;
public class Solution
{
public int[] TwoSum(int[] nums, int target)
{
Dictionary numDict = new Dictionary();
for (int i = 0; i < nums.Length; i++)
{
int complement = target - nums[i];
if (numDict.ContainsKey(complement))
{
return new int[] { numDict[complement], i };
}
if (!numDict.ContainsKey(nums[i]))
{
numDict[nums[i]] = i;
}
}
throw new ArgumentException("No two sum solution");
}
}
Step 5: Explain the Code
The code above works in the following way:
- First, it initializes the Dictionary.
- It iterates through the array, calculating the complement for each number.
- If the complement exists in the Dictionary, it returns the indices.
- If it does not exist, it stores the current number and its index in the Dictionary.
Complexity Analysis
Time Complexity: O(n), as it only traverses the array once.
Space Complexity: O(n), in the worst case, as we may have to store all numbers in the Dictionary.
Conclusion
Through this problem, we implemented an efficient algorithm in C#. Since various problems appear in coding tests, it is recommended to practice consistently. It is important to maintain your skills even after resigning!
In the next post, we will cover different types of problems. Stay tuned!