UVA Problem 10696 – f91 Solution

UVA Problem 10696 – f91 Solution:


Click here to go to this problem in uva Online Judge.

Solving Technique:

This is a simple DP problem (OR, so it looks but there’s a better way). It can be solved both iteratively and recursively with or without memoization. Still it will get accepted. But still the runtime is slow due huge input. So using a faster io may help it get accepted with a better rank.

For the matter of practice i will show all four methods i know of to solve this problem. Codes are given in this order,

  1. dynamic programming solution.
  2. Recursive solution without Memoization.
  3. Recursive solution with Memoization.
  4. Shortcut Trickery.
  5. Maybe extend the shortcut Trickery with memoization or other technique by yourself ? try it !!
Before starting to look below take look at this codes ( Fibonacci DP, Memoization ) with explanation.

Solution explanation:

Here the formula is given, we just need rearrange it and get the recurrence relation,

If N ≤ 100, then f91(N) = f91(f91(N+11));
If N ≥ 101, then f91(N) = N-10.

So from the given statement generating recurrence relation will be,

f(n) = { f(f(n+11)), if n <= 100
       { n - 10,     if n >= 101

Here base case is for any n greater than or equal to 101 is return n – 10. No more recursive call on that.
This can be simplified but i won’t go into that. The codes below will give a better idea.

Now, just create a function named f and use if else or ternary to implement the given relation and it’ll work.

Memoization:

Since we’ve found out the recurrence relation it won’t be hard to memoize the code. We can use a memo array to store the already calculated results. We need not change much from the previous recursive solution to memoize it.

Add an if condition in the beginning if the value is already calculated and stored in memo array just return. Other wise when returning just store the return values in memo.

That’s it.

Dynamic Programming ( bottom up method ):

Our recurrence relation will never change. Since there is no other function needed we just calculate and store values in memo array. I started from 1,000,000 because the problem statement said an integer may be at most 1,000,000. Just loop through that many times and store results in memo array.

Last Solution Trick:

It come to me when i was testing my own inputs. Anything less or equal 100 result in 91. Any greater value results in that value minus 10. Take a look at the code below to get the idea.

Important:  Be sure to add or print a new line after each output unless otherwise specified. The outputs should match exactly because sometimes even a space character causes the answer to be marked as wrong answer. Please compile with c++ compiler as some of my codes are in c and some in c++.


Input:

500
91
1
5
190
100
101
0

 


Output:

490
91
91
91
180
91
91

Code Bottom Up (Iterative) Dynamic Programming:

/**
 * @author  Quickgrid ( Asif Ahmed )
 * @link    https://quickgrid.wordpress.com
 * Problem: UVA 10696 - f91
 * Type:    Bottom Up (Iterative) Dynamic Programming
 */

#include<cstdio>
#include<sstream>

using namespace std;

#define N 1000000

static int F[N];

int main(){
    int n, i;

    for(i = N; i >= 0; --i){
        if(i >= 101)
            F[i] = i - 10;
        else
            F[i] = F[F[i + 11]];
    }

    while(scanf("%d", &n) && n)
        printf("f91(%d) = %d\n", n, F[n]);

    return 0;
}

Code Top Down (Recursive) Without Memoization:

/**
 * @author  Quickgrid ( Asif Ahmed )
 * @link    https://quickgrid.wordpress.com
 * Problem: UVA 10696 - f91
 * Type:    Top Down (Recursive) without Memoization
 */

#include<stdio.h>

int f(int n){
    return (n >= 101) ? n - 10 : f(f(n + 11));
}

int main(){
    int n;

    while(scanf("%d",&n) && n)
        printf("f91(%d) = %d\n", n, f(n));

    return 0;
}

Code Top Down (Recursive) with Memoization:

/**
 * @author  Quickgrid ( Asif Ahmed )
 * @link    https://quickgrid.wordpress.com
 * Problem: UVA 10696 - f91
 * Type:    Top Down (Recursive) with Memoization
 */

#include<stdio.h>

static unsigned F[1000000];

unsigned f(unsigned n){
    if(F[n])
        return F[n];

    if(n >= 101)
        return F[n] = n - 10;
    else
        return F[n] = f(f(n+11));
}

int main(){
    unsigned n;

    while(scanf("%u",&n) && n)
        printf("f91(%u) = %u\n", n, f(n));

    return 0;
}

Shortcut Technique:

/**
 * @author  Quickgrid ( Asif Ahmed )
 * @link    https://quickgrid.wordpress.com
 * Problem: UVA 10696 - f91
 * Type:    shortcut technique
 */

#include<stdio.h>

int main(){
    unsigned n;

    while(scanf("%u", &n) && n){
        if(n <= 100)
            printf("f91(%d) = 91\n", n);
        else
            printf("f91(%d) = %d\n", n, n - 10);
    }

    return 0;
}

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