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# Logic for solving Guess It Right problem on CodeChef ?

 0 1 https://www.codechef.com/problems/GUESSRT The problem was asked on febuary 2019 long challenge.I was able to solve the problem partially using two loops but i got TLE for last two test cases. Can someone share the right approach.Thanks Basically - two approach are possible - 1.Chef removes the boxes after every move (if it is not last move ). 2.or greedy approach. I am new to Competitive programming.Also i tried this problem for long time. asked 14 Feb, 16:28 0●2 accept rate: 0%

 4 answered 15 Feb, 16:29 371●6 accept rate: 0% 1 @rananjay23 here's a solution of your answer (15 Feb, 16:30) 1 if you don't understand anything feel free to ask (15 Feb, 16:31) Thanks @rajput1999 (15 Feb, 22:15)
 3 Hi @jbhv12 ! I modified your solution and submitted. It got an AC. See here. Mistakes i found in your code:- 1) Instead of power you used inbuilt pow function many times. That would obviously result in overflow and give wrong value. I replaced every pow with your power(). 2) I replaced your following line ll p = (pow(n,y+1)) - (n*pow(n-1,y)) + (pow(n-1,y)); with ll p = (power(n,y+1, mod))%mod; p -= (n*power(n-1,y, mod))%mod; p += mod; p %= mod; p += (power(n-1,y, mod))%mod; p %= mod; Because whenever you subtract something then you may get negative value so taking modulo will also result in negative answer. So always do ans = (ans+mod)%mod. 3) I replaced your line ll p1 = a*q + p*b; with ll p1 = (a*q)%mod + (p*b)%mod; 4) and put %mod on almost every step. Hooray! It resulted in AC. General Tips for such questions: 1) Always use modulo power exponentiation whenever answer is asked to compute modulo M. 2) Whenever ans becomes negative(either by subtracting or by multiplying with negative number) use ans = (ans%MOD + MOD) % MOD as suggested by @l_returns 3) Use %MOD after each arithmetic operation. answered 14 Feb, 21:15 5★vichitr 265●5 accept rate: 11% correction : 2) Whenever ans becomes negative(either by subtracting or by multiplying with negative number) use ans = (ans%Mod + MOD) % MOD (15 Feb, 22:25)
 1 If CHEF has more than 1 move then he will first remove boxes then he will choose one box. This leads to a GP $\frac{1}{n} + (1- \frac{1}{n})*\frac{1}{n}+(1- \frac{1}{n})^2*\frac{1}{n}+....$ If $m$ is odd then GP would have $\frac {m}{2}$ terms. If m is even then still GP would have same $\frac {m}{2}$ terms and also a last term would be ${(1- \frac{1}{n})}^{m/2}*\frac{1}{n+k}$. So summation of this GP is the answer. If $m$ is ODD then answer is $\frac{1}{n} + (1- \frac{1}{n})*\frac{1}{n}+(1- \frac{1}{n})^2*\frac{1}{n}+...+{(1- \frac{1}{n})}^{m/2}*\frac{1}{n}$ Else ($m$ is EVEN) answer is $\frac{1}{n} + (1- \frac{1}{n})*\frac{1}{n}+(1- \frac{1}{n})^2*\frac{1}{n}+...+{(1- \frac{1}{n})}^{m/2-1}*\frac{1}{n} + {(1- \frac{1}{n})}^{m/2}*\frac{1}{n+k}$ answered 14 Feb, 18:04 5★vichitr 265●5 accept rate: 11% @vichitr I figured out exact same logic. Here's my code getting WA: https://www.codechef.com/viewsolution/22948884 I've been pulling my hair. help me? (14 Feb, 19:53) jbhv122★ @vichitr Thanks for answer , it was really helpful .How do i prove that removing the boxes after each move is best option. obviously its feels that it should be but how do i prove it.Thanks (15 Feb, 14:00)
 0 @rajput1999 could you please explain from scratch (atleast name the recommended topics) answered 15 Feb, 22:29 0●1 accept rate: 0% i am a newbie please help me (15 Feb, 22:31) It is just a simple maths problem which uses the knowledge of Geometric Progression. Also you need knowledge to find modular multiplicative inverse. (16 Feb, 03:23) vichitr5★ Thanks man @vichitr ! i appreciate your support ... this is a very nice explanation it's a simple GP i was so confused (16 Feb, 16:03)
 0 i am a newbie please help me answered 15 Feb, 22:31 0●1 accept rate: 0% check out my already given 2 answers. Logic is explained and you then need to find GP sum as described in my answer. (16 Feb, 03:26) vichitr5★ Thanks man @vichitr ! i appreciate your support ... this is a very nice explanation it's a simple GP i was so confused (16 Feb, 16:04)
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question asked: 14 Feb, 16:28

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last updated: 17 Feb, 16:41