RAYS Coding

RAYS Coding

6/5/2025, 7:15:20 PM

Now, let’s move to the next Interview Coding Pattern in the Python Learning Series: For those of you who recently joined this channel, you should first go through: Python Learning Series: https://whatsapp.com/channel/0029VaiM08SDuMRaGKd9Wv0L/1527 Interview Coding Patterns: https://whatsapp.com/channel/0029VaiM08SDuMRaGKd9Wv0L/1629 *Two Pointer Technique* *Why interviewers ask this?* The Two Pointer technique is very common in array and string problems, especially when you're trying to optimize for time complexity. It's often used in: - Sorted arrays - Finding pairs/triplets with certain conditions - Merging arrays - String manipulation problems It shows the interviewer that you can write optimized solutions without brute force. *Use this technique when:* You have to compare or pair elements in a sequence. You're solving problems like: - Pair sum in sorted array - Reversing strings or arrays - Moving zeros to the end - Sorting squares of a sorted array - Checking for palindromes *Types of Two Pointer Techniques:* *1. Opposite Direction:* Start one pointer at the beginning and one at the end, then move toward the center. Example: Finding if a pair in a sorted array adds up to a target. *2. Same Direction:* Start both pointers from the beginning and move forward. Example: Removing duplicates, merging arrays. Let’s discuss some important coding interview problems: *Problem: Pair with Target Sum (in sorted array)* > Given a sorted array of integers and a target value, return whether there exists a pair of numbers that adds up to the target. *Example:* arr = [1, 2, 4, 6, 10] target = 8 *Approach:* Use one pointer from the start and one from the end. Move them based on the current sum. *Code*: def has_pair_with_sum(arr, target): left = 0 right = len(arr) - 1 while left < right: current_sum = arr[left] + arr[right] if current_sum == target: return True elif current_sum < target: left += 1 else: right -= 1 return False *Explanation:* left starts from the beginning; right from the end. You calculate current = arr[left] + arr[right]. If current == target: You've found the pair! If current < target: Move left forward to increase the sum. If current > target: Move right backward to decrease the sum. Keep moving until pointers meet. If no pair is found, return False. *Why this works better than brute force?* Brute force = O(n²) Two pointers = O(n) You avoid checking every pair. *2. Remove Duplicates from Sorted Array* *Problem:* Given a sorted array, remove the duplicates in-place such that each element appears only once. *Example:* [1, 1, 2, 2, 3] → [1, 2, 3] *Approach:* Use two pointers — one for placing the unique value, and the other for traversing. *Code:* def remove_duplicates(nums): if not nums: return 0 slow = 0 for fast in range(1, len(nums)): if nums[fast] != nums[slow]: slow += 1 nums[slow] = nums[fast] return slow + 1 # length of unique elements *3. Reverse a String or Array* *Problem:* Reverse a string or array in-place. *Example:* ["h", "e", "l", "l", "o"] → ["o", "l", "l", "e", "h"] *Code:* def reverse_string(s): left, right = 0, len(s) - 1 while left < right: s[left], s[right] = s[right], s[left] left += 1 right -= 1 *4. Move Zeros to the End* *Problem:* Move all zeros in an array to the end while maintaining the order of non-zero elements. *Example:* [0, 1, 0, 3, 12] → [1, 3, 12, 0, 0] *Code:* def move_zeros(nums): non_zero = 0 for i in range(len(nums)): if nums[i] != 0: nums[non_zero], nums[i] = nums[i], nums[non_zero] non_zero += 1 *5. Check if a String is a Palindrome* *Problem:* Check if a string is the same when reversed. *Example:* "madam" → True, "racecar" → True, "hello" → False *Code:* def is_palindrome(s): left, right = 0, len(s) - 1 while left < right: if s[left] != s[right]: return False left += 1 right -= 1 return True *6. Sorted Squares of a Sorted Array* *Problem:* Return the squares of a sorted array, also sorted. *Example:* [-4, -1, 0, 3, 10] → [0, 1, 9, 16, 100] *Approach:* Two pointers from both ends — place the larger square at the end. Code: def sorted_squares(nums): result = [0] * len(nums) left, right = 0, len(nums) - 1 index = len(nums) - 1 while left <= right: if abs(nums[left]) > abs(nums[right]): result[index] = nums[left] ** 2 left += 1 else: result[index] = nums[right] ** 2 right -= 1 index -= 1 return result *Two Pointer technique is one of the easiest ways to optimize problems involving arrays or strings — especially when working with sorted data or search-based problems.* *React with ❤️ once you're ready for next quiz on this topic*