SIGSEGV Error in Simple Dynamic Programming on SPOJ - c++

Question

I am just beginning with dynamic programming, and I have just attempted a simple question based on DP, on Spoj. Link - http://www.spoj.com/problems/MST1/
Here is the question statement -
On a positive integer, you can perform any one of the following 3
steps.
1.) Subtract 1 from it. ( n = n - 1 )
2.) If its divisible by 2, divide by 2. ( if n % 2 == 0 , then n = n / 2 )
3.) If its divisible by 3, divide by 3. ( if n % 3 == 0 , then n = n / 3 )
Given a positive integer n and you task is find the minimum number of
steps that takes n to one.
Input:
The input contains an integer T (1 ≤ T ≤ 100) number of test cases.
Second line input is N (0 < N ≤ 2*10^7 ) that indicates the positive
number.
Output:
For each case, print the case number and minimum steps.
Here's my code -
#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
using namespace std;
// Memo Function returns the smallest number of steps possible for integer a
int memo(int a, int mem[]);
int mem[20000010];
int main() {
int t;
scanf("%i", &t);
for(int i = 1; i <= t; i++) {
int n;
scanf("%i", &n);
memset(mem, -1, sizeof(mem));
mem[1] = 0;
printf("Case %i: %i\n", i, memo(n, mem));
}
return 0;
}
int memo(int a, int mem[]) {
if (mem[a] != -1) return mem[a]; // If the value of smallest steps have already been calculated
int r; // Current Lowest number of steps
r = memo(a - 1, mem) + 1;
if (a % 2 == 0) r = min(r, memo(a/2, mem) + 1);
if (a % 3 == 0) r = min(r, memo(a/3, mem) + 1);
mem[a] = r;
return r;
}
I have looked up this error on the internet and here on StackOverflow, and I have found that it may occur when we are trying to access the memory that has not been allocated, for example accessing the 11th element of a 10 element array. But I don't think that's the case here.
Also, I think the upper limit of the question is 2*10^7, also the array is global, so it shouldn't be an issue. Maybe there's some issue in the way I am using the memset function? I really don't know!
Any help will be appreciated! Thanks for reading!

Answer 1

Your DP idea is correct but your code is not working for a large inputs (e.g. 1x10^6, or the upper boundary, 2x10^7).
By changing your code a little, you can pre-compute every answer and then output only the ones you are interested in. It will be not very time-consuming because of the dynamic programming fashion of the problem, i.e., a complex problem can be solved as a combination of one or more previously solved problems.
int main()
{
// Initialize DP array
memset(mem, -1, sizeof(mem));
mem[1] = 0;
// Pre-compute every possible answer
for(int i = 2; i <= 20000000; i++)
mem[i] = memo(i);
// Read the number of test cases
int t;
scanf("%d", &t);
// Print only the desired answer, ie, mem[n]
for(int i = 1; i <= t; i++) {
int n;
scanf("%d", &n);
printf("Case %d: %d\n", i, mem[n]);
}
return 0;
}
I got an ACCEPTED with this approach.
Another tip: since your DP array is global, you do not have to pass it to the DP function every time.