How to Check if a String is a Palindrome

Q: How would you implement a function to check if a string is a palindrome using a data structure of your choice?

  • Data Structures And Algorithms
  • Mid level question
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Understanding how to determine if a string is a palindrome is a fundamental skill in coding interviews and competitive programming. A palindrome is a word, phrase, or sequence that reads the same backward as forward. Classic examples include words like "radar" and "level." This concept transcends mere whimsicality and dives deep into string manipulation techniques and data structures.

Candidates should know several methods to tackle this problem, as interviewers often look for efficiency and clarity in solutions. In software engineering, knowing how to manipulate strings is essential. Various common data structures, such as arrays, stacks, and queues, can be employed to achieve the desired functionality. For instance, using a stack can provide a straightforward mechanism for reversing a string and comparing it to the original.

This lends the candidate an opportunity to showcase an understanding of last-in, first-out principles inherent in stack operations. Moreover, candidates should also be familiar with the concept of time and space complexity. Implementing a function to check for palindromes can be done in linear time, O(n), which is optimal as it ensures each character in the string is evaluated only once. Evaluators appreciate candidates who not only provide correct solutions but also articulate their thought processes concerning optimization.

Understanding why a certain approach is more efficient than another can distinguish a good candidate from an excellent one. Additionally, candidates should consider edge cases, such as handling special characters, spaces, and case sensitivity when determining if a string is a palindrome. This not only demonstrates thoroughness but also critical thinking capability, traits highly valued in technical interviews. Overall, the palindrome-checking function is frequent in interviews as it encapsulates key programming concepts, making mastery of this topic crucial for prospective tech candidates. Engaging with similar problems can provide invaluable practice and build confidence for real-world coding scenarios..

To check if a string is a palindrome, I would use a stack data structure. A palindrome is a string that reads the same forwards and backwards, so by utilizing a stack, we can effectively reverse the string and compare it to the original.

Here’s how I would implement the function in Python:

```python
def is_palindrome(s: str) -> bool:
# Normalize the string by removing non-alphanumeric characters and converting to lower case
normalized_str = ''.join(char.lower() for char in s if char.isalnum())

# Use a stack to reverse the normalized string
stack = []

# Push all characters of the normalized string onto the stack
for char in normalized_str:
stack.append(char)

# Pop characters from the stack and compare with the normalized string
for char in normalized_str:
if char != stack.pop():
return False

return True
```

### Clarification:
1. Normalization: The function first normalizes the input string by removing any non-alphanumeric characters and converting it to lower case to ensure the comparison is case-insensitive and ignores spaces or punctuation.
2. Stack Usage: The stack stores the characters of the normalized string. When we pop characters off the stack, they come off in reverse order, which allows us to compare this reversed order with the original normalized string.
3. Time Complexity: This approach runs in O(n) time, where n is the length of the string, as we traverse the string a couple of times (once for normalization and once for comparisons).
4. Space Complexity: The space complexity is also O(n) due to the additional stack that stores the characters of the string.

### Examples:
- For the input "A man, a plan, a canal: Panama", the function would return `True`.
- For the input "race a car", the function would return `False`.