Got this question in a recent interview. Basic String compare with a little twist. I have an input String, STR1 = 'ABC'. I should return "Same/Similar" when the string to compare, STR2 has anyone of these values - 'ACB' 'BAC' 'ABC' 'BCA' 'CAB' 'CBA' (That is same characters, same length and same no of occurrences). The only answer struck at that moment was to proceed with 'Merge sort' or 'Quick Sort' since it's complexity is logarithmic. Is there any other better algorithm to achieve the above result?
Sorting both, and comparing the results for equality, is not a bad approach for strings of reasonable lengths.
Another approach is to use a map/dictionary/object (depending on language) from character to number-of-occurrences. You then iterate over the first string, incrementing the counts, and iterate over the second string, decrementing them. You can return false as soon as you get a negative number.
And if your set of possible characters is small enough to be considered constant, you can use an array as the "map", resulting in O(n) worst-case complexity.
Supposing you can use any language, I would opt for a python 'dictionary' solution. You could use 2 dictionaries having as keys each string's characters. Then you can compare the dictionaries and return the respective result. This actually works for strings with characters that appear more than once.
Related
I encountered the following problem for which I couldn't quite find the appropriate solution.
The problem says for a given string having a specific hash value, find the lowest string (which is not the same as the given one) of the
same length and same hash value (if one exists). E.g. For the
following value mapping of alphabets: {a:0, b:1, c:2,...,z:25}
If the given string is: ady with hash value - 27. The
lexicographically smallest one (from all possible ones excluding the
given one) would be: acz
Solution approach I could think of:
I reduced the problem to Coin-Change problem and resorted to finding all possible combinations for the given sum. Out of all the obtained solutions, I sort them up and find the lowest (or the next smallest if the given string is smallest).
The problem however lies with finding all possible solutions (even in a DP approach) which might be inefficient for larger inputs.
My doubt is:
What solution strategy (possibly even Greedy) could give a better time complexity than above?
I cannot guarantee that this will give you a lower complexity, but a couple of things:1 you don't need to check all the space, just the space of lexicographic value less than or equal to the given string. 2: you can formulate it as an integer programming problem:
Assuming your character space is the letters, and each letter is given its number index[0-25] so a corresponds to 0, b to 1 and so forth. let x_i be the number of letters in your string corresponding to index i. You can formulate your problem as:
min sum_i(wi*xi)
st xi*ai = M
xi>=0,
sum_i(xi)=n
sum_i(wi*xi)<= N
xi integer
Where wi= 26^i, ai is equal to hash(letter(i)), n is the number of letters of the original string, N is the hash value of the original string. This is an integer programming problem so you can try plugging it to a solver. The original problem is very similar to subset sum problem with fixed subset size (where the hash values are the elements you are summing over, and the subset size is the length of the string) so you might also want to take a look at that, although as you will see from the answer it is a complicated problem.
I have a bunch of strings in an array that I have defined in the data segment. If I were to take 2 of the strings from the array, is it possible to compare them to see which has a greater value in mips? How would I do this? Basically, I'm looking to rearrange the strings based on alphabetical order.
EDIT: This is less of me trying to get help with a specific problem, and more of just a general question that will help me with my approach to the code. Thanks!
If it were me, I'd create a list of pointers to the strings. That is, a list of the addresses of each string. Then you'd write a subroutine the compares two strings given their pointers. Then, when you need to swap the strings, you simply swap the actual pointers.
You want to avoid swapping the strings themselves, since they may well be tightly packed, thus you'd have to do a lot of shifting to move the holes of memory around. Pointers are simple to swap. You could swap strings more easily if they were all of a fixed length (or less), then you wouldn't have to worry about moving the memory holes around.
But sorting the pointer list is really the hot tip.
To compare strings, the simplest way is to iterate over each character of each string, and subtract them from each other. If the result is 0, they're equal. If not, then if the result is > 0, then the first string is before the other string, otherwise the second string is lower and you would swap them. If you run out of either string before the other, and they're equal all the way to that point, the shorter string is less than the longer one.
I have one question to ask on Algorithms. I have been asked to write the algorithm on this: Not asking you to write the algo for me, but just let me know the efficient process what I need to do:
There is an array of n elements like the book or contents of Bible, and Suppose you have inserted a input string "Gaurav Agarwal" in that. What you want to do you need to fetch unique elements that are present in the array for that String. Just an algorithm how you will proceed further (unsorted)
If you did not understand then let me know and I will try to help on this.
One good way to find duplicates in an unsorted array is to sort it based on the string elements, therefore the algorithm for your homework question would be:
Sort the array
check your array for existence of "Gaurav Agarwal". Since it is sorted, neighboring elements would be the same string, and what you need to do then is to keep a counter and increment it until you find the first array element that is not equal to the string you're looking for
it will take some time to sort the string array and then to parse it. I would recommend just to parse the array of string and verify if length of your string is the same as the length of the string from the current position of the array. If the length is the same, compare the 2 strings
I dont think sorting and searching is the most efficient solution to your problem.
Sorting itself has nlogn complexity.
Just doing a bruteforce search of array is more efficient(has a complexity of n)
This is the case if you are finding unique elements for one string or a few strings.
If you are trying to find unique elements for a lot of input strings instead of one only then sorting makes sense.
I would proceed in the following steps:
I would use a hash table with chaining, using a hash function that
works well for strings.
find the hash of the new string and search the linked list of the
slot corresponding that hash for duplicates.
so i have a list of string
{test,testertest,testing,tester,testingtest}
I want to sort it in descending order .. how do u sort strings in general ? Is it based on the length or is it character by character ??
how would it be in the example above ?? I want to sort them in a descending way.
No matter what language you’re in, there’s a built-in sort function that performs a lexicographical order, which returns
['test','tester','testertest','testing','testingtest']
for your example. If I wanted this reversed, I would just say reversed(sorted(myList)) in Python and be done with it. If you look to your right you can see plenty of related questions that require a more specialized ordering method (for numbers, dates, etc.), but lexicographic order works on strings containing any kind of data.
Here’s how it works:
compare(string A, string B):
if A and B are both non-empty:
if A[0] == B[0]:
// First letters are the same; compare by the rest
return compare(A[1:], B[1:])
else:
// Compare the first letters by Unicode code point
return compare(A[0], B[0])
else:
// They were equal till now; the shorter one shall be sorted first
return compare(length of A, length of B)
I would sort it like this:
testingtest
testing
testertest
tester
test
Assuming C#
string[] myStrings = {"test","testertest","testing","tester","testingtest"};
Array.Sort(myStrings);
Array.Reverse(myStrings);
foreach(string s in myStrings)
{
Console.WriteLine(s);
}
Not always an ideal way to do it - you could implement a custom comparer instead - but for the trivial example you asked about this is probably the most logical approach.
In computer science strings are usually sorted character by character, with the preferred sort order being (for a standard english character set):
Null characters first
Followed by whitepsace
Followed by symbols
Followed by numeric characters in obvious numerical order
Followed by alphabetic characters in obvious alphabetical order
When sorting characters generally lowercase characters come before uppercase characters.
So for example if we were to sort / compare:
test i ng
test e r
Then "tester" would come before "testing" - the first different character in the string is the 5th one, and "e" comes before "i".
Similarily if we were to compare:
test
testing
Then in this case "test" would come first - once again the strings are identical until the 5th character, where the string "test" ends (i.e. no character) which becomes before any alphanumerical character.
Note that this can produce some counter-intutive results when dealing with numbers - for example try sorting the strings "50" and "100" - you will find that "100" comes before "50". Why? because the strings differ at character 1 and "5" comes after "1".
In nearly all languages there is a function which will do all of the above for you!
You should use that function instead of trying to sort strings yourself! For example:
// C#
string[] myStrings = {"test","testertest","testing","tester","testingtest"};
Array.Sort(myStrings);
in Java you can use natural ordering with
java.util.Collections.sort(list);
the make it descending
java.util.Collections.reverse(list);
or create your own Comparator to do the reverse sorting.
When comparing two strings to see which sorts first, the comparison is typically done on a character by character basis. If the characters in the first position (e.g., t in your example) are identical, you move to the next character. When two characters differ, that "may" define which string is considered "greater".
However, depending on the locale used and a number of other factors, it is possible for later characters in the two strings being compared to override a difference in an earlier character. For example, in some collations, the diacritics on letters are considered to be of secondary weight. So a primary difference in a later character can override the secondary difference.
When two strings are otherwise identical but one is longer, the longer one is typically considered to be "greater". When sorting in descending order, the "greater" of two strings is sorted first.
Do you want to know if test should appear after tester in a descending order? Or are you particularly interested in sorting strings with similar prefixes?
If it's the later, I'd suggest a Trie if the input tends to grow big time.
Proposed answer:
Strings are simply arrays of characters so the O-notation will be dependent on the number of characters in the string (if the loop depends on the length of the string). In this case the O-notation wouldn't be affected because the length of the string is a constant.
Any other ideas? Am I reading this question correctly?
This is not true, since representing integers in arrays are not boundless.
IOW a string that represents an 32-bit integer is maximally 32-bit, thus maximally 10 digits in base 10, and O(10) is a negiable constant that doesn't change the O notation.
So, in summary, while strings are O(n), basic integer types represented as strings are O(maximally 10)=O(0)
I think you need to specify your problem better.
Try thinking about something that operates on an array of integers or an array of strings, clearly in the latter case you have an array of array of a primitive type rather than an array of a primitive type. How does this change things?
That depends entirely on what you are doing with the strings.
If you for example copy items from one array to another, the result is depending on the implementation. It's still an O(n) operation, but the meaning of n changes. If copying a string causes a new copy to be created, n means the total number of characters in all the strings. If copying a string is only copying the reference to it, n means the total number of strings.