Two pointer pattern in DSA
Hey there! Let's chat about this cool trick called the two-pointer technique in DSA. Don't worry, I'll keep it fun and throw in some visuals to help it stick. Ready to dive in?
So, what's this two-pointer thing all about?
Think of it like a game where you've got two players (we'll call them pointers) starting on different sides of a field (that's your array). They can either:
- Run towards each other (kinda romantic, right?)
- Race in the same direction (getting competitive!)
- Do their own thing (freestyle mode)
This technique helps you solve a bunch of problems super efficiently without writing a ton of loops. Pretty neat, huh?
Why should you care about it?
Well, it's like a superpower for your code:
- It's fast: Solves problems in O(n) instead of O(n²). Your code will zoom!
- It's simple: Fewer lines, easier to understand.
- It's flexible: Works with arrays, strings, even linked lists!
Let's look at some types of two-pointer problems
- Pointers Moving Toward Each Other
Imagine you're trying to find two numbers in a sorted array that add up to a target. It's like two people running towards each other to meet in the middle.
Here's a quick JavaScript example:
function twoSumSorted(arr, target) { let left = 0; let right = arr.length - 1; while (left < right) { const sum = arr[left] + arr[right]; if (sum === target) return [left, right]; if (sum < target) left++; else right--; } return -1; // No pair found } console.log(twoSumSorted([1, 2, 3, 4, 6], 10)); // Output: [2, 4]
Picture the numbers as cute little characters in a line:
① ② ③ ④ ⑤
- Left pointer starts at ①
- Right pointer starts at ⑤
- They slowly move towards each other to find the perfect match
2.This is perfect for checking if a string is a palindrome. Picture two friends starting at the ends of a word, moving towards the middle and high-fiving if everything matches.
function isPalindrome(s) { let left = 0; let right = s.length - 1; while (left < right) { if (s[left] !== s[right]) return false; left++; right--; } return true; } console.log(isPalindrome("racecar")); // Output: true console.log(isPalindrome("hello")); // Output: false
Imagine two ants crawling towards each other on the word "racecar":
r <-> r ?
a <-> a ?
c <-> c ?
Palindrome confirmed! ?
Some cool applications of this technique:
- Finding a target sum (like we did above)
- Merging two sorted arrays
- Calculating trapped rainwater (Google this one, it's fascinating!)
- Reversing linked lists
Pro tips:
- Sorting first can make these problems way easier
- Watch out for edge cases (empty arrays, duplicates, extreme values)
- Sketch it out! Drawing the array or string can help you avoid bugs
Want to level up? Try these challenges:
- Two Sum II - Input Array Is Sorted (LeetCode 167)
- Longest Substring Without Repeating Characters (LeetCode 3)
- Valid Palindrome (LeetCode 125)
- Trapping Rain Water (LeetCode 42) - if you're feeling adventurous!
The two-pointer technique is like a Swiss Army knife for coding. It's simple but powerful, and with some practice, you'll be using it without even thinking.
Got questions or want to share your solutions? Drop a comment or give me a shout. Happy coding!
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