PROBLEM LINK:Author: Prateek Gupta DIFFICULTY:Simple PREREQUISITES:String processing PROBLEM:Given a string $S$, can you remove at most one character so that the remaining string is a double string? A double string is a string of the form $W + W$ for some nonempty string $W$. QUICK EXPLANATION:The answer is
EXPLANATION:Subtask 1Let's first try using the simplest solution possible: Try all possible characters to remove (possibly none), and see whether any resulting string is a double string. There are only $S+1$ cases to try, because there are only $S$ locations to remove a character from, plus $1$ for the "do nothing" scenario. For this to work, we need code that checks whether a string is a double string. Here's a pseudocode that does that:
(Note: We also return With this function, we can now answer a single test case by trying all $S+1$ cases. The following is a pseudocode for this:
Unfortunately, checking whether a string is a double string takes $O(S)$ time, so this runs in $(S+1)O(S) = O(S^2)$ time, too slow for the second subtask. Subtask 2Here are a few observations that will help us find a faster solution:
This gives us the following clues on what we must do:
In the first case, since we can't remove any character, we simply need to check whether $S$ is already a double string to begin with. This takes $O(S)$ time so we're good. In the second case, we still have a problem because we don't know which letter to remove. However, we can use the fact that in a double string, the first half is the same as the second half (by definition). Since we can only remove one character to turn $S$ into a double string, the first "half" of $S$ must already be nearly the same as the second "half" to begin with. (The word "half" here is a bit ambiguous because $S$ is odd, but you sorta get the idea.) Specifically, the first and second "halves" must differ from each other by exactly one deletion. This means that the shorter "half" must be a subsequence of the longer "half". Thus, we have reduced the problem to simply checking whether some string is a subsequence of another string! Now, we still have a problem because we don't know which "half" must be the shorter one. But there are only two possibilities to check (either the first or second half is the shorter one), so it's no problem. Finally, how do we quickly check whether some string, say $A$, is a subsequence of another string, say $B$? A simple greedy algorithm such as the following will do:
This code runs in $O(A + B)$ time, and since in our case $A + B = S$, and there are only two cases to check, our algorithm runs in $2\cdot O(S) = O(S)$. To summarize, here is our algorithm:
Time Complexity:$O(S)$ AUTHOR'S AND TESTER'S SOLUTIONS:
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asked 13 Mar '16, 14:17

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answered 15 Mar '16, 18:54

It was so easy to miss the obvious with this problem. It is unsurprising that the test cases were initially inadequate. The note from the editorial made my error apparent to me: "Note: We also return false when n == 0 because W must be nonempty from the definition!" answered 15 Mar '16, 17:00

Simple string processing....MySolution answered 15 Mar '16, 15:45

thought the same way nice question https://www.codechef.com/viewsolution/9578821 answered 15 Mar '16, 15:48

Can someone please help me and tell where is the problem in my code? I am getting WA for last test cases of both sub tasks. answered 15 Mar '16, 18:29

same logic but got wrong answer!!! https://www.codechef.com/viewsolution/9674515 answered 15 Mar '16, 18:35
Try this 1 cxccc Answer  YES
(15 Mar '16, 23:32)
corrected the error https://www.codechef.com/viewsolution/9685962 still not working
(16 Mar '16, 22:19)

https://www.codechef.com/viewsolution/9678465 can anyone tell me for what testcase this fails ?? because it works for every testcase i can think of and get only the last testcase of second subtask wrong answered 15 Mar '16, 20:20

can any one please tell me where My Solution is failed answered 15 Mar '16, 23:28

Very easy problem. Consider all the possible cases. Check out my sol https://www.codechef.com/viewsolution/9655225 answered 16 Mar '16, 11:01

Getting a wrong answer for my solution :https://www.codechef.com/viewsolution/9683501...dono for what it fails! Can someone look at it? answered 16 Mar '16, 14:23

I tried all test cases given in the comments.. My code gives correct answer for all of them, but it fails for last test case in both sub tasks. Can someone please check what the error is in my code.? https://www.codechef.com/viewsolution/9688335 Thanks in advance... answered 17 Mar '16, 10:27

can someone plz post the testcases . i have written the code but ans is always showing to be wrong!! here is the link to my code ! http://ideone.com/Q5Wyyw i dont get why codechef submission is always wrong!! answered 17 Mar '16, 20:23

Nice editorial. Breaking and showing all the operations in the form of functions will definitely inculcate better coding practices among the newbies