Day 75: 
Mastering HashMaps for Problem Solving in DSA
If you’re diving into coding interviews or just want to level up your DSA skills, HashMaps (aka Hash Tables or Dictionaries) are a tool you need in your toolkit. They’re fast, flexible, and surprisingly fun to use once you get the hang of them.
Why HashMaps Matter
HashMaps let you store key-value pairs and access them in O(1) time. That’s a game-changer when you’re trying to solve problems quickly and efficiently.
Think of them as smart lockers—you instantly know where the key is without checking every locker.
Common Use Cases
Counting elements
→ E.g. {‘a’: 2, ‘b’: 1}
For problems like “First Unique Character”
Tracking indices
→ For “Two Sum”, store value → index
Avoiding repeated work
→ Use it in memoization during recursion
Example Problem: Two Sum (Classic)
Problem: Given an array and a target, return the indices of two numbers that add up to the target.
js
CopyEdit
const twoSum = (nums, target) => {
const map = {};
for (let i = 0; i < nums.length; i++) {
const diff = target - nums[i];
if (map[diff] !== undefined) {
return [map[diff], i];
}
map[nums[i]] = i;
}
};
Uses a HashMap to check for the complement in O(1)
Pro Tip
Always ask yourself:
“Can I reduce the time complexity if I use a map to track values?”
Bonus Challenge to Try
Find the length of the longest substring without repeating characters.
Hint: You’ll need a HashMap AND a sliding window 
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