Two Sum

easy

Find two numbers that add up to target using hash map

Two Sum

Key Insight

Use hash map to store complements. For each num, check if (target - num) exists.

Step 1Setup
Find two numbers summing to 9
Array
2
0
7
1
11
2
15
3
HashMap
Empty

Create empty hash map to store {value: index}. Target = 9.

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Learn the Pattern

Two PointersUse two pointers to traverse an array or string, reducing time complexity from O(n^2) to O(n) for problems involving pairs or subarrays.
Hash MapUse a hash map for O(1) lookup to track seen elements, frequencies, or complement values during array traversal.
Practice the Code

Step-by-Step Walkthrough: Two Sum

Use hash map to store complements. For each num, check if (target - num) exists.

  1. Setup

    Create empty hash map to store {value: index}. Target = 9.

  2. Check First Element

    num=2. Complement = 9-2 = 7. Not in map. Store {2: 0}.

  3. Check Second Element

    num=7. Complement = 9-7 = 2. Found at index 0!

  4. Result

    Return indices [0, 1]. nums[0] + nums[1] = 2 + 7 = 9.