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Two Pointers & Sliding Window

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Two Sum Pattern

2 + 6 = 8

L=0target: 10R=9

Valid Palindrome

Clean the string, then use two pointers converging from both ends. Compare characters one pair at a time — any mismatch immediately rules out a palindrome.

Visualization

Input:

Speed

Characters Checked

Checking...

Result

Cleaned string: "amanaplanacanalpanama"

L=R

–

Left pointer (L)Right pointer (R)Matched pairMismatch

Ready — press Play or step through.

Complexity

O(n)

Best Case

O(n)

Worst Case

O(n)

Average

O(1)

Space

Space is O(1) extra if we use two pointers on the original string without creating a cleaned copy. The O(n) cleaning pass is unavoidable but the pointer traversal itself is O(1) space.

How It Works

1Strip non-alphanumeric characters and convert to lowercase
2Place left pointer at index 0, right pointer at the last index
3Compare s[left] and s[right]. If equal, move both inward. If not, return False
4When left >= right all pairs have matched — return True

Active Recall Quiz

Question 1 of 3

Why do we skip non-alphanumeric characters when checking for a palindrome?

DSAverse

Two Pointers & Sliding Window

Loading Two Pointers...

Two Sum Pattern

2 + 6 = 8

L=0target: 10R=9