JavaScript Coding Test Course, Two Pointers

Let’s take a closer look at the Two Pointer technique, which is a common algorithm that appears in coding tests. This course will explain the basic concept of the Two Pointer method and solve actual problems using it.

1. What is Two Pointer?

The Two Pointer technique is an algorithmic method that efficiently processes data in data structures like arrays or lists by using two pointers. Generally, it is effective in reducing unnecessary loops and improving time complexity as the size of the problem increases.

  • Efficiency: It often reduces the time complexity to O(N).
  • Simplicity: The code becomes simpler and more readable.
  • Application Scope: It can be used in various situations such as sorted arrays, strings, and subarray problems.

2. Basic Idea of Two Pointer

Two pointers are generally used in two ways:

  • Left and Right Pointers: Start from both ends of an array and move towards the center to find elements that satisfy the condition.
  • Moving in the Same Direction: Both pointers move in the same direction while examining surrounding elements until a specific condition is met.

3. Actual Problem: Two Sum

Here is an actual problem using the Two Pointer technique.

Problem Description

Given a sorted array numbers and a target sum target, write a function that returns the indices of the two numbers such that they add up to the target sum. Assume that each input has exactly one solution, and you cannot use the same element twice.

Input Format

numbers: [2, 7, 11, 15]
target: 9

Output Format

[0, 1]

Example Explanation

In the above example, since 2 + 7 = 9, the output is the indices 0 and 1.

4. Problem Solving Process

Let’s solve this problem using the Two Pointer method. Proceed with the following steps:

Step 1: Initialize Pointers

Initialize pointers at the start and end of the array. Name the left pointer as left and the right pointer as right.

Step 2: Check Conditions

Use a while loop to repeat until the two pointers cross each other. In each iteration, calculate the sum of the two numbers pointed to by the current pointers and check if this sum equals target.

Step 3: Compare Sum

  • If the sum is less than target, move the left pointer one step to the right.
  • If the sum is greater than target, move the right pointer one step to the left.
  • If the sum is equal to target, return the two indices.

Step 4: Code Implementation

function twoSum(numbers, target) {
        let left = 0; 
        let right = numbers.length - 1;

        while (left < right) {
            const sum = numbers[left] + numbers[right];

            if (sum === target) {
                return [left, right]; 
            } else if (sum < target) {
                left++; 
            } else {
                right--; 
            }
        }
        return []; // In case there is no answer
    }

Step 5: Code Analysis

The time complexity of this code is O(N), and the space complexity is O(1). That means it can solve the problem without storing the array additionally.

5. Additional Examples and Variations

Now, let’s look at other variation problems. This can be applied to finding all combinations of three numbers that add up to target from the given array.

Problem Description

Given an integer array numbers and an integer target, return the indices of all unique combinations of three numbers that sum up to target.

Input Format

numbers: [1, 2, 3, 4, 5]
target: 9

Output Format

[[0, 3, 4], [1, 2, 4], [2, 3, 4]]

Solution Approach

To solve this problem, we can add a second pointer to find combinations without duplicates. The modified algorithm is as follows:

function threeSum(numbers, target) {
        let result = [];
        numbers.sort((a, b) => a - b); // Sort the array

        for (let i = 0; i < numbers.length - 2; i++) {
            let left = i + 1;
            let right = numbers.length - 1;

            while (left < right) {
                const sum = numbers[i] + numbers[left] + numbers[right];

                if (sum === target) {
                    result.push([i, left, right]);
                    left++;
                    right--;
                    while (left < right && numbers[left] === numbers[left - 1]) left++; // Remove duplicates
                    while (left < right && numbers[right] === numbers[right + 1]) right--; // Remove duplicates
                } else if (sum < target) {
                    left++;
                } else {
                    right--;
                }
            }
        }
        return result;
    }

6. Conclusion

The Two Pointer technique is a very useful method for processing arrays or lists. It can significantly improve performance, especially when dealing with sorted data. Through the content covered in this course, I hope you gain an understanding of the basic concepts and applications of Two Pointers, and help you solve real-world problems.

Continue practicing various situations where you can use Two Pointers in search and combination problems, and confidently apply them in actual coding interviews.

Practice Problems

Try the following problems:

  • Given an integer array numbers and an integer target, return the combination of indices where the sum of the two closest numbers is equal to target.
  • A problem that returns the length of a substring consisting of unique characters within a string.

By solving problems like these, you can enhance your understanding of the Two Pointer technique and gain experience in solving various problems.