codechef: A puzzle game - c++

Question

problem statement:
Johnny has some difficulty memorizing the small prime numbers. So, his computer science teacher has asked him to play with the following puzzle game frequently.
The puzzle is a 3x3 board consisting of numbers from 1 to 9. The objective of the puzzle is to swap the tiles until the following final state is reached:
1 2 3
4 5 6
7 8 9
At each step, Johnny may swap two adjacent tiles if their sum is a prime number. Two tiles are considered adjacent if they have a common edge.
Help Johnny to find the shortest number of steps needed to reach the goal state.
My solution so far
#include<bits/stdc++.h>
using namespace std;
bool prime[20];
int matrix[3][3];
int solved[3][3] = {
{1,2,3},
{4,5,6},
{7,8,9}
};
void display()
{
for(int row = 0; row<3;row++)
{
for(int col = 0;col<3;col++)
{
cout<<matrix[row][col]<<" ";
}
cout<<endl;
}
cout<<endl<<endl;
}
bool check(){
for(int i=0;i<3;i++){
for(int j=0;j<3;j++){
if(matrix[i][j]!=solved[i][j])
return false;
}
}
return true;
}
int min(int a,int b)
{
return (a<b)?a:b;
}
void generate(){
memset(prime,true,sizeof(prime));
for(int i=2;i*i<20;i++){
if(prime[i]==true)
{
for(int j=2*i;j<20;j+=i)
prime[j]=false;
}
}
}
int getMoves(int row, int col){
if(row < 0 ||col< 0 || row>=3||col>=3){
return 0;
}
if(check()){
return 0;
}
int moves = 0;
for(int i = row-1 ; i<= row+1 ;i++)
{
for(int j = col -1 ; j<=col+1;j++)
{
if((i!=row-1&&j!=col-1)||(i!=row+1&&j!=col+1)||(i!=row+1&&j!=col-1)||(i!=row-1&&j!=col+1)){
if(prime[matrix[row][col]+matrix[i][j]]==true)
{
moves+=getMoves(i,j);
int temp;
temp = matrix[i][j];
matrix[i][j] = matrix[row][col];
matrix[row][col] = temp;
display();
}
}
}
}
return moves;
}
int Moves(){
int minMoves = INF;
for(int row = 0;row<3;row++)
{
for(int col = 0;col<3;col++)
{
int moves = getMoves(row,col);
minMoves = min(moves,minMoves);
}
}
return minMoves;
}
int main(){
generate();
int t;
cin>>t;
while(t--)
{
for(int row = 0; row<3;row++)
{
for(int col = 0;col<3;col++)
{
cin>>matrix[row][col];
}
}
}
cout<<Moves();
}
sample testcase
Input:
2
7 3 2
4 1 5
6 8 9
9 8 5
2 4 1
3 7 6
Output:
6
-1
the program keeps crashing I guess because of memory overflow issue.

Answer 1

if (row < 0 || col< 0 || row >= 3 || row <= 3) {
return 0;
}
The code after this part is 'not accessible' because this condition is always true (... row >= 3 || row <= 3). You probably meant to write: (... row >= 3 || col >= 3)

Answer 2

I'm afraid your code is completely wrong and I don't think it could by fixed without complete rewrite. For example in function getMoves() your variables i and j can acquire value -1 so you will face access violation error. Secondly you have a recursion there, but you don't change data before you invoke recursion. Let's assume you want to swap 7 and 4. In the next step (because you didn't change input) you can swap 4 and 1. But it's not a correct move, because in that time, 4 should't be there. Thirdly your function getMoves() can end in an endless loop.
In conclusion, these kinds of problem are solved quite differently. You can for exmaple use backtracking algorithm or you can use A* algorithm. You will have to evaluate your current state. Let assume the following state:
7 3 2
4 5 6
1 8 9
You can measure number of moves that the number has to do to go to its correct position. So in this case 1 has to do 2 moves, 7 has to do 2 moves, 2 has to do 1 move as well as the number 3. The value of this state is 2 + 2 + 1 + 1 = 6. It's called an heuristic function. Now you can take this function and put it in an A* algorithm, and you should see the correct result.