Valid Palindrome Two Pointer in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the left pointer at the start of the string.

DSA C

int left = [1];

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A0

B1

Cstrlen(s)

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Starting left pointer at 1 skips the first character.

Using strlen(s) causes out-of-bounds access.

The left pointer should start at index 0 to begin checking from the start of the string.

2fill in blank

medium

Complete the code to initialize the right pointer at the end of the string.

DSA C

int right = [1] - 1;

Drag options to blanks, or click blank then click option'

A0

Bstrlen(s)

C1

Dsizeof(s)

Attempts:

3 left

💡 Hint

Common Mistakes

Using sizeof(s) returns size of pointer, not string length.

Setting right to 0 starts at the beginning, not the end.

The right pointer should start at the last character index, which is length minus one.

3fill in blank

hard

Fix the error in the while loop condition to continue while left is less than right.

DSA C

while ([1] < right) {

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Aright - 1

Bright

Cleft + 1

Dleft

Attempts:

3 left

💡 Hint

Common Mistakes

Using right < right causes no iterations.

Using left + 1 < right is incorrect syntax.

The loop should run while the left pointer is less than the right pointer.

4fill in blank

hard

Fill both blanks to skip non-alphanumeric characters from the left and right pointers.

DSA C

while (left < right && !isalnum(s[[1]])) left++;
while (left < right && !isalnum(s[[2]])) right--;

Drag options to blanks, or click blank then click option'

Aleft

Bright

C0

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using fixed indices like 0 or 1 ignores pointer positions.

Mixing left and right in wrong places causes logic errors.

We check characters at the current left and right indices to skip non-alphanumeric characters.

5fill in blank

hard

Fill all three blanks to compare characters ignoring case and move pointers accordingly.

DSA C

if (tolower(s[[1]]) != tolower(s[[2]])) {
    return false;
}
[3];
[4];

Drag options to blanks, or click blank then click option'

Aleft

Bright

Cleft++

Dright--

Attempts:

3 left

💡 Hint

Common Mistakes

Not using tolower() causes case-sensitive mismatches.

Forgetting to move pointers causes infinite loops.

We compare characters at left and right ignoring case. If they differ, return false. Otherwise, move left forward and right backward.