Master the Two Pointer Technique: With 10 Practice Problems

What is the Two Pointer Technique?

The two pointer technique is a pattern where you use two pointers (indices) to traverse a data structure, typically an array or linked list. It's incredibly powerful for solving problems that would otherwise require nested loops (O(n^2)) in O(n) time.

When to Use Two Pointers

Use two pointers when you see:

Types of Two Pointer Approaches

1. Opposite Direction (Start/End)

Both pointers start at opposite ends and move toward each other.

def two_sum_sorted(arr, target):

left, right = 0, len(arr) - 1

while left < right:

current_sum = arr[left] + arr[right]

if current_sum == target:

return [left, right]

elif current_sum < target:

left += 1

else:

right -= 1

return []

2. Same Direction (Fast/Slow)

Both pointers start at the same position but move at different speeds.

def has_cycle(head):

slow = fast = head

while fast and fast.next:

slow = slow.next

fast = fast.next.next

if slow == fast:

return True

return False

3. Sliding Window (Special Case)

A variation where you maintain a window of elements.

def max_sum_subarray(arr, k):

window_sum = sum(arr[:k])

max_sum = window_sum

for i in range(k, len(arr)):

window_sum = window_sum - arr[i-k] + arr[i]

max_sum = max(max_sum, window_sum)

return max_sum

10 Practice Problems

Here are 10 problems to master two pointers (in order of difficulty):

Common Mistakes to Avoid

Time Complexity

Most two pointer solutions run in O(n) time and O(1) space, which is why interviewers love them. They test if you can optimize brute force solutions.

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