INSQ15_G - Editorial

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Author : Dhruv Chandhok

Tester : Shekhar Nain

Editorialist: Dhruv Chandhok

DIFFICULTY

Medium

PREREQUISITES

Game-Theory , Staircase Nim

PROBLEM

Various people are standing on various blocks in a rectangular matrix. You can select a block with atleast 1 person and push any positive number of people from that block to the block above or right of that. You have to push all the people to the top-right corner.

EXPLANATION

This question uses the concept of "staircase nim" of game theory.

Let’s start from the basic.

P1(Dassu) moves first. Suppose that there is a person at (i,j) and he has to reach (x,y). If (x+y) and (i+j) has the same parity, after reaching (x,y) it would be P1’s move. So if (x+y) and (i+j) has the same parity, there’s no change in order to continue the game forward.
However if they have different parity, the order would change.

We Can assign a value 0 or 1 to the situation.

0 - same parity i.e no change in order

1 - different parity i.e. change in order

Now suppose there are 2 people and let us suppose that their moves are independent. So we will consider the 2 independently and assign 0 or 1 to each as described above. To combine these independent situation into one, we can XOR them. Why this works?

Now we need to consider one more case. If there are more than one person in a block, they are not independent. Why? Because we can move them all (or some) in one move to the next block. So what we need to see is the least (non-negative)number that is not possible when we make a move on the current block. Grundy Numbers

In case , there was only a single person, we can move only one - so remaining can be 0. So minimum not possible is 1.

In case , there were 2 people, we can move one or both - so remaining can be 0,1. So minimum not possible is 2.

In case , there were 3 people, we can move 1,2 or all- so remaining can be 0,1,2So minimum not possible is 3.

Hence in this example grundy number of a particular pile( or block ) is the number of people in the block.

Now we have considered all the cases:-
So what has our solution reduced to?

ans=0
for( all the blocks )
 {
    if parity_current_block is not equal to parity_destination_block
    ans= ans ^ grundy_number;
 }

Alternate Approach

The (1,M) is a loosing case. If all are on (1,M) , the player who makes the first move looses. So the Grundy Number assigned to (1,M) is 0.

We can go to (1,M) only from (1,M-1) and (2,M). So their grundy number are 1 as we can reach a 0 condition from there.

For(2,M-1), we can reach 1-condition but not 0-condition. So its grundy number becomes 0.
Proceeding in this way, We see that grundy number possible are 0 & 1 and are alternation.

For N=4 and M=3, grundy number matrix would look like:-

0 1 0

1 0 1

0 1 0

1 0 1

The person standing on a particular block are it’s multiplier.

ans=0
for( all the blocks )
  {
    grundy_number = ( (i+j)%2 != (1+m)%2 )
    ans= ans ^ ( grundy_number * multiplier );
  }

Setter Code


[6]


In case the range for Input i.e. N & M are small, the following code would work :- 

    #include<bits/stdc++.h>
    using namespace std;
     
    #define LL long long int
    vector<vector<int> > a;
    int N,M,P;
    void play()
    {
        int grundy_value, ans = 0; 
        LL i,j;
        for(i=N;i>=1;i--)
        {
          for(j=1;j<=M;j++)
          {
             grundy_value=a[i][j];
             if( (i+j) % 2 != (1+M) % 2) //(1,M) is the destination
             ans^=grundy_value;
          }
        }
        cout<<(ans ? "Yes" : "No")<<endl;
    }
     
    int main() 
    {
        int i,j;
        cin>>N>>M>>P;
        a.resize(N+1,vector<int> (M+1,0));
        for(i=0;i<P;i++)
        {
        	int X,Y;
        	cin>>X>>Y;
        	a[X][Y]++;
        }
        play();
        return 0;
    }

But as the range grows, we need to think of alternate methods rather than storing the people as matrix.

The Following code will work :- 

    #include<bits/stdc++.h>
    using namespace std;
     
    #define LL long long int
    vector<pair<LL,LL> > a;
    LL N,M,P;
    void play()
    {
    	sort(a.begin(),a.end());
        LL grundy_value=1, ans = 0;
        LL i;
        for(i=1;i<P;i++)
        {
             if(a[i]==a[i-1])
             grundy_value++;
             else
             {
              	LL x,y;
              	x=a[i-1].first;
              	y=a[i-1].second;
              	if( (x+y) % 2 != (1+M) % 2) //(1,M) is the destination
             	ans^=grundy_value;
             	grundy_value=1;
             }
        }
        		LL x,y;
              	x=a[P-1].first;
              	y=a[P-1].second;
              	if( (x+y) % 2 != (1+M) % 2) //(1,M) is the destination
             	ans^=grundy_value;
     
        cout<<(ans ? "Yes" : "No")<<endl;
    }
     
    int main()
    {
        LL i,j;
        cin>>N>>M>>P;
        for(i=0;i<P;i++)
        {
        	LL X,Y;
        	cin>>X>>Y;
        	a.push_back(make_pair(X,Y));
        }
        play();
        return 0;
    }
    

### TESTER SOLUTION



7

2 Likes

Nice Editorial

1 Like

nice editorial.

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