Add Two Sum

Author: thanthtooaung-coding

README.md

@@ -12,16 +12,21 @@ Each problem has its own directory containing:
 
 | # | Title | Difficulty | Solution | Topics |
 |---|-------|------------|----------|---------|
+| 1 | [Two Sum](Two%20Sum) | Easy | [Java](Two%20Sum/Solution.java) | Array, Hash Table |
 | 2924 | [Find Champion II](Find%20Champion%20II) | Medium | [Java](Find%20Champion%20II/Solution.java) | Graph |
 
 ## Categories
 
 ### By Difficulty
-- Easy: 0
+- Easy: 1
 - Medium: 1
 - Hard: 0
 
 ### By Topics
+- Array Problems
+  - [Two Sum](Two%20Sum)
+- Hash Table Problems
+  - [Two Sum](Two%20Sum)
 - Graph Problems
   - [Find Champion II](Find%20Champion%20II)
 

Two Sum/README.md

@@ -0,0 +1,74 @@
+# Two Sum
+
+## LeetCode Problem
+- Problem Number: 1
+- Problem Link: https://leetcode.com/problems/two-sum/
+- Difficulty: Easy
+- Topics: Array, Hash Table
+
+## Problem Description
+Given an array of integers `nums` and an integer `target`, return indices of the two numbers in the array that add up to the `target`.
+
+You may assume that:
+- Each input has exactly one solution
+- You may not use the same element twice
+- You can return the answer in any order
+
+## Examples
+
+### Example 1:
+```
+Input: nums = [2,7,11,15], target = 9
+Output: [0,1]
+Explanation: Because nums[0] + nums[1] == 9, we return [0, 1].
+```
+
+### Example 2:
+```
+Input: nums = [3,2,4], target = 6
+Output: [1,2]
+```
+
+### Example 3:
+```
+Input: nums = [3,3], target = 6
+Output: [0,1]
+```
+
+## Constraints
+- 2 <= nums.length <= 10⁴
+- -10⁹ <= nums[i] <= 10⁹
+- -10⁹ <= target <= 10⁹
+- Only one valid answer exists
+
+## Solution Approach
+There are two main approaches to solve this problem:
+
+### 1. Hash Table Approach (Optimal)
+- Time Complexity: O(n)
+- Space Complexity: O(n)
+- Algorithm:
+  1. Create a HashMap to store complement values
+  2. For each number in the array:
+     - Calculate complement (target - current number)
+     - If complement exists in HashMap, return [complementIndex, currentIndex]
+     - Add current number and its index to HashMap
+
+### 2. Brute Force Approach
+- Time Complexity: O(n²)
+- Space Complexity: O(1)
+- Algorithm:
+  1. Use nested loops to try every possible pair
+  2. Check if any pair sums to target
+  3. Return indices when found
+
+## Common Pitfalls
+1. Using the same element twice
+2. Not considering negative numbers
+3. Forgetting to check if complement exists before adding to HashMap
+4. Returning wrong order of indices
+
+## Related Problems
+- 167: Two Sum II - Input Array Is Sorted
+- 170: Two Sum III - Data structure design
+- 653: Two Sum IV - Input is a BST

Two Sum/Solution.java

@@ -0,0 +1,19 @@
+import java.util.HashMap;
+
+class Solution {
+    public int[] twoSum(int[] nums, int target) {
+        HashMap<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);
+        }
+        
+        return new int[] {};
+    }
+}