Solving the Two Sum Problem with Java: A Step-by-Step Guide

The two sum problem is a common coding challenge that involves finding two numbers in an array that add up to a specific target sum. This problem can be solved using a variety of algorithms and data structures, including brute force, hash tables, and sorting. In this blog, we will demonstrate how to solve the two sum problem using a hash table in Java.

Problem Statement: Given an array of integers, return the indices of the two numbers such that they add up to a specific target.

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Solution:

1. Create a hash table to store the elements of the array as keys and their indices as values.
2. Loop through each element in the array, and calculate the difference between the target sum and the current element.
3. Check if the difference is present in the hash table, and return the indices of the two elements if it is.
4. If the difference is not present in the hash table, add the current element and its index to the hash table.
5. If no two elements are found that add up to the target sum, return an error message.

Here is the implementation of the solution in Java:

csharpCopy codeclass Solution {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            int complement = target - nums[i];
            if (map.containsKey(complement)) {
                return new int[] { map.get(complement), i };
            }
            map.put(nums[i], i);
        }
        throw new IllegalArgumentException("No two sum solution");
    }
}

Explanation:

1. We first create a hash table called map to store the elements of the array as keys and their indices as values.
2. In the for loop, we loop through each element in the array, and calculate the difference between the target sum and the current element.
3. We use the containsKey method to check if the difference is present in the hash table. If it is, we return the indices of the two elements.
4. If the difference is not present in the hash table, we use the put method to add the current element and its index to the hash table.
5. If no two elements are found that add up to the target sum, we throw an error message using the IllegalArgumentException.

Conclusion:

The two sum problem is a common coding challenge that can be solved using various algorithms and data structures, including hash tables. By using a hash table, we can efficiently find the two numbers in an array that add up to a specific target sum. If you have any questions or feedback, feel free to leave a comment below.