Hello everyone!

Today I want to explain one of the most useful techniques in competitive programming — Two Pointers * What is Two Pointers?

Two pointers is a technique where we use two indices to iterate over an array or string.

Usually:

• One pointer starts from the beginning
• Another from the end (or also from beginning)

• Example Problem

Given an array, find if there exist two numbers whose sum equals k.

• Idea

1. Sort the array

2. Set:

   left = 0 right = n — 1

3. While left < right:

   if a[left] + a[right] == k → FOUND if sum < k → left++ if sum > k → right--

• Code (C++)

cpp

include <bits/stdc++.h>

using namespace std;

int main() { int n, k; cin >> n >> k; vector a(n); for(int i = 0; i < n; i++) cin >> a[i]; sort(a.begin(), a.end()); int l = 0, r = n — 1; while(l < r) { int sum = a[l] + a[r]; if(sum == k) { cout << "YES\n"; return 0; } else if(sum < k) l++; else r--; } cout << "NO\n"; } * Complexity

Sorting: O(n log n) Two pointers: O(n)

Total: O(n log n)

• When to Use?

1) Sorted arrays 2)Pair problems 3)Subarray problems

*Final Tip

If you see:

1) “find pair” 2) “sum equals k” 3) “continuous segment” Think about Two Pointers

Thanks for reading! If this helped you, please upvote