AlgoDaily - Validate Palindrome

Good evening! Here's our prompt for today.

Given a string str, can you write a method that will return True if is a palindrome and False if it is not? If you'll recall, a palindrome is defined as "a word, phrase, or sequence that reads the same backward as forward". For now, assume that we will not have input strings that contain special characters or spaces, so the following examples hold:

1let str = 'thisisnotapalindrome';
2isPalindrome(str);
3// false
4
5str = 'racecar';
6isPalindrome(str);
7// true

For an extra challenge, try to ignore non-alphanumerical characters. The final solution that we present will handle all edge cases.

Constraints

Length of the given string <= 100000
The string will consist of ASCII characters (some or all)
Expected time complexity : O(n)
Expected space complexity : O(1)

Try to solve this here or in Interactive Mode.

How do I practice this challenge?

xxxxxxxxxx
​
 
package main
​
import (
  "fmt"
)
​
func isPalindrome(s string) bool {
  // fill in this function
  return true
}
​
func main() {
​
  fmt.Println(isPalindrome("A man, a plan, a canal: Panama"))
​
  // test cases start
  fmt.Printf("==============================================================\n")
  passed := true
  if isPalindrome("A Santa Lived As a Devil At NASA") == true {
    fmt.Println("TEST 1 --- PASSED")
  } else {
    fmt.Println("TEST 1 --- FAILED : got ", isPalindrome("A Santa Lived As a Devil At NASA"), " expected true")
    passed = false
  }
  if isPalindrome("gold") == false {
    fmt.Println("TEST 2 --- PASSED")
  } else {
    fmt.Println("TEST 2 --- FAILED : got ", isPalindrome("gold"), " expected false")
    passed = false

Tired of reading? Watch this video explanation!

To change the speed of the video or see it in full screen, click the icons to the right of the progress bar.

Here's our guided, illustrated walk-through.

How do I use this guide?

Build your intuition. Is this statement true or false?

A string is defined as a palindrome if the reversal of the string is equal to the original string.

For example, “toot” is a palindrome, but “boot” is not.

Press true if you believe the statement is correct, or false otherwise.

This is a classic question, and there are multiple ways to solve this. For the sake of learning, let's cover all of them!

Using built-in methods

This would probably be invalid in an actual interview, but you can rely on the built-in String method to accomplish a quick reversal. In Javascript, you can simply call reverse() and in Python, you can call [::-1] You can then compare the reversed string to the original:

xxxxxxxxxx
 
package main
​
import (
  "fmt"
  "strings"
)
​
func isPalindrome(s string) bool {
  // Calling reverse function
  runes := []rune(s)
  for i, j := 0, len(runes)-1; i < j; i, j = i+1, j-1 {
    runes[i], runes[j] = runes[j], runes[i]
  }
  reversed := string(runes)
​
  // Checking if both strings are equal or not
  if s == reversed {
    return true
  }
  return false
}
​
func main() {
  fmt.Println(isPalindrome("racecar"))
}

Try this exercise. Could you figure out the right sequence for this list?

What's the order of successfully finding out if a string is a palindrome?

Press the below buttons in the order in which they should occur. Click on them again to un-select.

Options:

Open a while loop to perform while low is less than high
Continue until end of loop and return true
Define two variables: high and low, as 0 and (length of string - 1)
If `string[low]` does not equal `string[high]`, return false. Increment low, decrement high

Try this exercise. Click the correct answer from the options.

What will the following pseudocode do to an input string?

1func reverseStr(str string) string {
2    strCopy := []rune(str)
3    start := 0
4    end := len(str) - 1
5    for start < end {
6        strCopy[start], strCopy[end] = strCopy[end], strCopy[start]
7        start++
8        end--
9    }
10    return string(strCopy)
11}

Click the option that best answers the question.

Make a copy
Reverse the string
Swap the first and last letters
Infinite loop

With a while loop:

We can cut down on the number of operations by recognizing that we don't need to do len(str)-1 iterations. Instead of using just one pointer that simply iterates through the string from its end, why not use two?

xxxxxxxxxx
 
package main
​
import (
  "fmt"
)
​
func isPalindrome(s string) bool {
  left := 0
  right := len(s) - 1
  for left < right {
    if s[left] != s[right] {
      return false
    }
    left++
    right--
  }
  return true
}
​
func main() {
  fmt.Println(isPalindrome("racecar"))
}

What we're doing above is specifying two pointers, start and end. start points to the beginning of the string, and end is a pointer to the last character. Taking the example input racecar, as we run through it, these are the comparisons we'll see:

xxxxxxxxxx
 
racecar
^     ^
racecar
 ^   ^
racecar
  ^ ^
racecar
   ^
True

Build your intuition. Click the correct answer from the options.

What is the run time of the following code?

PYTHON

1def reverse_str(str):
2  start = 0
3  end = len(str)-1
4  str_copy = [letter for letter in str]
5  while start < end:
6    temp = str_copy[start]
7    str_copy[start] = str_copy[end]
8    str_copy[end] = temp
9    start += 1
10    end -= 1
11  return "".join(str_copy)

Complexity of Final Solution

Let n be the length of the string. We iterate n/2 times until the left pointer is greater than the right pointer for linear O(n) time complexity, and we use a constant O(1) space for the 2 pointers.

Click the option that best answers the question.

O(log n)
O(n)
O(n log n)
O(n^2)

One Pager Cheat Sheet

You can write a method using O(n) time and O(1) space complexity to check if a given string of length 100000 or less is a palindrome, taking into account non-alphanumerical characters.
The given string is a palindrome if it reads the same forwards as it does backwards.
With built-in methods, you can easily reverse() or [::-1] a String and then compare it to the original to check if it is a palindrome.
By utilizing two variables, low and high, to represent the starting and ending indices of a string to be checked, and using a while loop to compare characters at these indices, we can determine if a string is a palindrome or not.
The code reverses a string by looping over the characters, swapping the characters and keeping track of the start and end indexes using a temp variable, and then joining the characters of the str_copy list to return the reversed string.
We can reduce complexity by using two pointers instead of one and only have to do len(str)/2 iterations of the while loop in order to validate if a string is a palindrome.
Comparing characters of a string with two pointers, start pointing to the beginning and end pointing to the last character, the example input racecar shows us the initial comparisons.
The runtime of the code is O(n), with O(1) space complexity, where n is the length of the string, since it iterates linearly with a constant space overhead.

This is our final solution.

To visualize the solution and step through the below code, click Visualize the Solution on the right-side menu or the VISUALIZE button in Interactive Mode.

xxxxxxxxxx
}
 
package main
​
import (
  "fmt"
  "strings"
)
​
func isPalindrome(s string) bool {
​
  s = strings.ToLower(s)
​
  i, j := 0, len(s)-1
  for i < j {
    for i < j && !isChar(s[i]) {
      i++
    }
    for i < j && !isChar(s[j]) {
      j--
    }
    if s[i] != s[j] {
      return false
    }
    i++
    j--
  }
​
  return true
}
​
func isChar(c byte) bool {
  if ('a' <= c && c <= 'z') || ('0' <= c && c <= '9') {
    return true

Alright, well done! Try another walk-through.

If you had any problems with this tutorial, check out the main forum thread here.

Constraints

Try to solve this here or in Interactive Mode.

Tired of reading? Watch this video explanation!

Here's our guided, illustrated walk-through.

Build your intuition. Is this statement true or false?

Using built-in methods

Try this exercise. Could you figure out the right sequence for this list?

Options:

Try this exercise. Click the correct answer from the options.

Click the option that best answers the question.

With a while loop:

Build your intuition. Click the correct answer from the options.

Complexity of Final Solution

Click the option that best answers the question.

One Pager Cheat Sheet

This is our final solution.

Alright, well done! Try another walk-through.

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