Ad 1: It is impossible to define reverse/2 as a (directly edit thx to @repeat: tail) recursive predicate – unless you permit an auxiliary predicate. Ad 2: palindrome(X) :- reverse(X,X). But the easiest way is to define such predicates with DCGs: iseq([]) –> []. iseq([E|Es]) –> iseq(Es), [E]. reverse(Xs, Ys) :- phrase(iseq(Xs), Ys). palindrome(Xs) :- … Read more
The Big Picture We will first look at this regex from the overall big picture algorithm, and then take a closer look at the specific implementation details later. The regex is an almost direct translation of the following Java code: static boolean isPalindrome(String s) { if (s.isEmpty()) { return true; } String g2 = null; … Read more
Just for fun: return myString.SequenceEqual(myString.Reverse());
This can be solved in O(n^2) using dynamic programming. Basically, the problem is about building the longest palindromic subsequence in x[i…j] using the longest subsequence for x[i+1…j], x[i,…j-1] and x[i+1,…,j-1] (if first and last letters are the same). Firstly, the empty string and a single character string is trivially a palindrome. Notice that for a … Read more
This seems like a lot of code. Have you tried a very naive approach yet? Checking whether something is a palindrome is actually very simple. private boolean isPalindrome(int possiblePalindrome) { String stringRepresentation = String.valueOf(possiblePalindrome); if ( stringRepresentation.equals(stringRepresentation.reverse()) ) { return true; } } Now that might not be the most performant code, but it gives … Read more
function palindrome(str) { var len = str.length; var mid = Math.floor(len/2); for ( var i = 0; i < mid; i++ ) { if (str[i] !== str[len – 1 – i]) { return false; } } return true; } palindrome will return if specified word is palindrome, based on boolean value (true/false) UPDATE: I opened … Read more
You can find the the longest palindrome using Manacher’s Algorithm in O(n) time! Its implementation can be found here and here. For input String s = “HYTBCABADEFGHABCDEDCBAGHTFYW1234567887654321ZWETYGDE” it finds the correct output which is 1234567887654321.
The answer to this question is that “it is impossible”. More specifically, the interviewer is wondering if you paid attention in your computational theory class. In your computational theory class you learned about finite state machines. A finite state machine is composed of nodes and edges. Each edge is annotated with a letter from a … Read more
Why not just: public static boolean istPalindrom(char[] word){ int i1 = 0; int i2 = word.length – 1; while (i2 > i1) { if (word[i1] != word[i2]) { return false; } ++i1; –i2; } return true; } Example: Input is “andna”. i1 will be 0 and i2 will be 4. First loop iteration we will … Read more
A pythonic way to determine if a given value is a palindrome: str(n) == str(n)[::-1] Explanation: We’re checking if the string representation of n equals the inverted string representation of n The [::-1] slice takes care of inverting the string After that, we compare for equality using ==