 # TREHUNT - Editorial

Author: Aryan Agarwala
Tester: Istvan Nagy
Editorialist: Aman Dwivedi

Easy - Medium

# PREREQUISITES:

Maths, Observations

# PROBLEM:

You are given an N \times M grid and an integer K, you have to find different pairs of cells such that the Manhattan distance between them is exactly K.

Let A_k be the number of desired pairs when the value of the Manhattan distance between the two cells is equal to K. Let C = \sum_{i=1}^{N+M-2} (A_i \times 31^{i-1}). You have to find the value of CC.

The answer may be large, so you need to find it modulo 998244353.

Note: The Manhattan distance between two points (x_1,y_1) and (x_2,y_2) is defined as |x_1−x_2|+|y_2−y_1|.

# EXPLANATION:

Let’s try to think of a brute force approach using some examples.

Let’s take N=2 and M=3.
Now following are the points between whom the distance is 1.

• (1,1) and (1,2)
• (1,2) and (1, 3)
• (2,2) and (2,3)
• (2 1) and (2,2)
• (1 1) and (2, 1)
• (1,2) and (2,2)
• (1,3) and (2,3)

The order in which the pair of points listed above have some reasons. Can you find it?

Hint

Look at the difference between their X and Y coordinates.

Let i be the distance in the X component then the difference in the Y component will be j where j= K - i.

If we take points:

• (1,1) and (1,2)
• (1,2) and (1, 3)
• (2,2) and (2,3)
• (2 1) and (2,2)

Then we can see that the difference between X and Y coordinates is 0 and 1 respectively.

And similary if we take points:

• (1 1) and (2, 1)
• (1,2) and (2,2)
• (1,3) and (2,3)

Then we can see that the difference between X and Y coordinates is 1 and 0 respectively.

Now we know that if we iterate i from 0 to K_l inclusive and sum the results then we can get A_{K_l} where 1 \leq K_l \leq N+M-2.

But what to add while iterating i from 0 to K_l to get A_{K_l}?

If we can observe that for the above point when the difference between X coordinates was 0 and the difference between Y coordinates was 1. Then we have a total of 4 pairs of points.

Therefore, we can say that there are N rows so (N - i) possibilities for the row of this cell. Now, the other cell has to be i units to the right, so the X coordinate for the other cell is fixed. And there are (M-j) possibilities for Y coordinate when it is above our first cell and if i \neq0 and j \neq 0, then we also have (M-j) possibilities if it is below our first cell.

Therefore we have (N-i) \times (M-j) \times 2 when i \neq0 and j \neq 0, else (N-i) \times (M-j) .

Pseudo Code - Brute Force
int mult(int a, int b) {
return (a * b) % MOD;
}
int add(int a, int b) {
return (a + b) % MOD;
}

int32_t main() {

int t;
cin >> t;
while (t--) {
int n, m;
cin >> n >> m;
int tr = 0;
int tm = 1;
for (int k = 1; k <= (n + m - 2); k++) {
int ans = 0;
for (int x = 0; x <= k; x++) {
int y = k - x;
if (x >= n || x < 0 || y >= m || y < 0) continue;
int ta = mult((n - x), (m - y));
if (x != 0 && y != 0) ta = mult(ta, 2);
}
tr += ((ans * tm) % MOD);
tr %= MOD;
tm *= 31;
tm %= MOD;
}
cout << tr << endl;
}
}


Now let’s try to optimize the Brute force method. In order to optimize we will replace the nested loop with some formula.
Currently inorder to calculate A_{K_l} for each i from 0 to K_l, with formula inside (N-i) \times (M-j). If we expand this formula we will get N \times (M-K) + (N-M-K) \times i + i^2.

• Now we can easily calculate (N-M-K) \times i = (N-M-K) \times \sum_{i=0}^{K}i = (N-M-K) \times \frac{ (i)*(i+1)}{2}.
• And \sum_{i=0}^{K}i^2 = \frac{i*(i+1)*(2*i + 1)}{6}

Now we remove the nested loop and get the sum in O(1).

# TIME COMPLEXITY:

O(N+M-2) per test case.

# SOLUTIONS:

Author
#include <bits/stdc++.h>
#define int long long
//#include <sys/resource.h>
#define initrand mt19937 mt_rand(time(0));
#define rand mt_rand()
#define MOD 1000000007
#define INF 1000000000
#define mid(l, u) ((l+u)/2)
#define rchild(i) (i*2 + 2)
#define lchild(i) (i*2 + 1)
#define mp(a, b) make_pair(a, b)
#define lz lazup(l, u, i);
using namespace std;
int mult(int a, int b){
return (a*b)%MOD;
}
return (a+b)%MOD;
}
int sub(int a, int b){
return (((a-b)%MOD)+MOD)%MOD;
}
int calcAns(int n, int m, int k, int x){
int tr = 0;
tr = add(tr, mult(x+1, mult(n, m)));
tr = sub(tr, mult(x+1, mult(n, k)));
tr = add(mult(mult(mult(x, x+1), 500000004), n), tr);
tr = add(mult(mult(mult(x, x+1), 500000004), k), tr);
tr = sub(tr, mult(mult(mult(x, x+1), 500000004), m));
tr = sub(tr, mult(mult(mult(x, x+1), 2*x + 1), 166666668));
tr = mult(tr, 2);
return tr;
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
int t;
cin>>t;
while(t--) {
int n, m;
cin>>n>>m;
int tr = 0;
int tm = 1;
for (int k = 1; k <= (n + m - 2); k++) { //we need k-i to be less than or equal to (m-1), implies
int minv = max(0ll, k - m + 1);
int maxv = min(k, n - 1);
int ans = calcAns(n, m, k, maxv);
if (minv > 0) ans = sub(ans, calcAns(n, m, k, minv - 1));
if (minv == 0) ans = sub(ans, mult(n, m-k));
if (minv <= k && maxv >= k) {
ans = sub(ans, mult(n - k, m));
}
tr += ((ans * tm) % MOD);
tr %= MOD;
tm *= 31;
tm %= MOD;
}
cout << tr << endl;
}
}


Tester
// created by mtnshh

#include<bits/stdc++.h>
using namespace std;
#define ll long long int
#define pb push_back
#define rb pop_back
#define ti tuple<int, int, int>
#define pii pair<int, int>
#define pli pair<ll, int>
#define pll pair<ll, ll>
#define mp make_pair
#define mt make_tuple

#define rep(i,a,b) for(ll i=a;i<b;i++)
#define repb(i,a,b) for(ll i=a;i>=b;i--)

#define err() cout<<"--------------------------"<<endl;
#define errA(A) for(auto i:A)   cout<<i<<" ";cout<<endl;
#define err1(a) cout<<#a<<" "<<a<<endl
#define err2(a,b) cout<<#a<<" "<<a<<" "<<#b<<" "<<b<<endl
#define err3(a,b,c) cout<<#a<<" "<<a<<" "<<#b<<" "<<b<<" "<<#c<<" "<<c<<endl
#define err4(a,b,c,d) cout<<#a<<" "<<a<<" "<<#b<<" "<<b<<" "<<#c<<" "<<c<<" "<<#d<<" "<<d<<endl

#define all(A)  A.begin(),A.end()
#define allr(A)    A.rbegin(),A.rend()
#define ft first
#define sd second

#define V vector<ll>
#define S set<ll>
#define VV vector<V>
#define Vpll vector<pll>

#define endl "\n"

long long readInt(long long l,long long r,char endd){
long long x=0;
int cnt=0;
int fi=-1;
bool is_neg=false;
while(true){
char g=getchar();
// char g = getc(fp);
if(g=='-'){
assert(fi==-1);
is_neg=true;
continue;
}
if('0'<=g && g<='9'){
x*=10;
x+=g-'0';
if(cnt==0){
fi=g-'0';
}
cnt++;
assert(fi!=0 || cnt==1);
assert(fi!=0 || is_neg==false);

assert(!(cnt>19 || ( cnt==19 && fi>1) ));
} else if(g==endd){
if(is_neg){
x= -x;
}
// cerr << x << " " << l << " " << r << endl;
assert(l<=x && x<=r);
return x;
} else {
assert(false);
}
}
}
string ret="";
int cnt=0;
while(true){
char g=getchar();
// char g=getc(fp);
assert(g != -1);
if(g==endd){
break;
}
cnt++;
ret+=g;
}
assert(l<=cnt && cnt<=r);
return ret;
}
long long readIntSp(long long l,long long r){
}
long long readIntLn(long long l,long long r){
}
}
}

const int max_q = 5;
const int max_n = 1e7;

const ll N = 200005;
const ll INF = 1e12;
const ll M = 998244353;

ll power(ll a,ll n,ll m=M){
ll ans=1;
while(n){
if(n&1) ans=ans*a;
a=a*a;
n=n>>1;
ans=ans%m;
a=a%m;
}
return ans;
}

void solve(ll n, ll m){
V ans(n+m);
ll inv_six = power(6, M-2, M);
rep(i,0,n+m-1){
ll cnt = i;
ll r_n = cnt, r_m = cnt, l_n = 0, l_m = 0;
if(r_n >= n){
r_n = n - 1;
l_m = cnt - r_n;
}
if(r_m >= m){
r_m = m - 1;
l_n = cnt - r_m;
}
if(l_m == 0){
l_m = 1;
r_n -= 1;
}
if(l_n == 0){
l_n = 1;
r_m -= 1;
}
if(l_n > r_n or l_m > r_m)
continue;
ll x = n - l_n;
ll y = m - r_m;
ll cnt_elem = (r_n - l_n);
ll res_1 = (x * y) % M;
ll res_2 = ((cnt_elem * (cnt_elem + 1) / 2)) % M;
ll res_3 = ((((cnt_elem * (cnt_elem + 1)) % M * (2 * cnt_elem + 1)) % M * inv_six)) % M;
ll z = ((cnt_elem + 1) * res_1 - (y - x) * res_2 - res_3) % M;
z = (z + M) % M;
cnt += 1;
ans[i] = 2*z;
}
rep(i,1,n){
ans[i] = (ans[i] + (n-i)*m) % M;
}
rep(i,1,m){
ans[i] = (ans[i] + n*(m-i)) % M;
}
ll final = 0, p = 1;
rep(i,1,n+m-1){
final = (final + (ans[i] * p)) % M;
p = (p * 31) % M;
}
cout << final << endl;
}

int main(){
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
ll sum_n = 0, sum_m = 0;
while(q--){

Nice explanation 