C# Coding Test Course, Calculating Continuous Sum

While preparing for coding tests or algorithm problem-solving, it is important to understand and learn approaches for various types of problems. Today, we will take an in-depth look at the problem of finding the sum of continuous elements.

Problem Description

We will examine the problem of calculating the sum of consecutive elements in a given integer array and finding the shortest continuous subarray that meets a specific criteria.

Problem Definition


Problem: Find the Sum of Consecutive Elements

Given an integer array nums and an integer target, 
return the length of the shortest continuous subarray whose sum is greater than or equal to target. 
If there is no such subarray, return 0.

Input Example:
nums = [2,3,1,2,4,3]
target = 7

Output Example:
2 (length of subarray [4,3])

Problem Solving Approach

To solve this problem, two main approaches can be used: brute force (exhaustive search) and the two-pointer (sliding window) method. Here, we will use the two-pointer method for an efficient solution.

Two-Pointer Approach

The two-pointer approach involves exploring the array using two pointers (left and right) to find the subarray that satisfies the desired condition. The advantage of this method is its time complexity of O(n), making it efficient.

Step-by-Step Solution

  1. Initialization: Initialize the two pointers. Set the left pointer left to 0 and the right pointer right to 0. Also, initialize the current sum currentSum to 0 and the minimum length minLength to infinity.
  2. Condition Check: Traverse the array using the right pointer and add nums[right] to currentSum. Then check if currentSum is greater than or equal to target.
  3. Adjusting the Subarray: If currentSum is greater than or equal to target, update the minimum length and increase the left pointer while subtracting nums[left] from currentSum. This helps find the shortest possible subarray.
  4. Termination Condition: Repeat this process until the right pointer reaches the end of the array.

Function Implementation

Now, let’s implement the above logic through C# code.


using System;

public class Solution {
    public int MinSubArrayLen(int target, int[] nums) {
        int left = 0;
        int currentSum = 0;
        int minLength = int.MaxValue;

        for (int right = 0; right < nums.Length; right++) {
            currentSum += nums[right];

            while (currentSum >= target) {
                minLength = Math.Min(minLength, right - left + 1);
                currentSum -= nums[left];
                left++;
            }
        }

        return minLength == int.MaxValue ? 0 : minLength;
    }
}

Code Explanation

The above C# code works as follows:

  • Initial Variable Setup: Initialize left, currentSum, and minLength.
  • Array Traversal: Traverse the array using the right variable and add the current element to currentSum.
  • Condition Check: If currentSum is greater than or equal to target, update minLength and increase the left pointer while subtracting nums[left] from currentSum.
  • Return Result: Finally, if minLength has not been updated, return 0; otherwise, return the found minimum length.

Example Test Cases

Now, let’s write some example test cases to test this function.


public static void Main(string[] args) {
    Solution sol = new Solution();
    
    Console.WriteLine(sol.MinSubArrayLen(7, new int[] { 2, 3, 1, 2, 4, 3 })); // Output: 2
    Console.WriteLine(sol.MinSubArrayLen(4, new int[] { 1, 4, 4 }));        // Output: 1
    Console.WriteLine(sol.MinSubArrayLen(11, new int[] { 1, 1, 1, 1, 1, 1 })); // Output: 0
    Console.WriteLine(sol.MinSubArrayLen(8, new int[] { 2, 3, 1, 2, 4, 3 })); // Output: 2
}

Conclusion

In this lecture, we learned the problem-solving approach using two pointers through the problem of finding the sum of consecutive elements. It is important to accurately understand the nature of a problem and choose the appropriate approach to solve algorithmic problems. In practice, solving various problems helps gain experience and become familiar with different algorithms, so continuous practice is encouraged.

Try to enhance your algorithmic sense by encountering and solving various problems!