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Михаил Долинский

Темы: 1438
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(Analysis by Dhruv Rohatgi )
In this problem, we're given a family tree and asked to identify the relationship between two cows; call them Bessie and Elsie. We will do this by finding the common ancestor of Bessie and Elsie (if she exists), and the number of generations by which each of Bessie and Elsie are removed from the ancestor cow. Notice that the numbers uniquely define the relationship between Bessie and Elsie.

But how do we find the two numbers? We can find the mother of a given cow in $O(N)$ time (also determining whether the mother even exists), by looping over all relationships. Call this the MOTHER function. Now we can determine whether one cow is a direct ancestor of another cow in $O(N^2)$ time, by repeatedly finding the mother of the second cow, and then her mother, and so forth. Let's call this the IS-ANCESTOR function. We can also have the IS-ANCESTOR function return the number of generations by which the first cow is removed from the second cow.

Now we almost have an algorithm. We can repeatedly check if Bessie is an ancestor of Elsie, and then if Bessie's mother is an ancestor of Elsie, and so forth. By keeping a counter, we can find the minimum number of generations up the family tree we need to go before Bessie's ancestor is an ancestor of Elsie. If no such ancestor satisfies the property, then we can immmediately output "NOT RELATED."

Otherwise, let $b$ be the distance of Bessie from the common ancestor, and let $a$ be the distance of Elsie from the common ancestor (where $a$ is returned by our IS-ANCESTOR function). All that remains is some casework, but it's possible to write less code by putting in a bit more thought. If $a$ and $b$ are both $1$, then Bessie and Elsie are "SIBLINGS." If $a$ and $b$ are both greater than $1$, then they are "COUSINS." Now observe that either $a > b$ or $b > a$. In the former case, Bessie is the mother/grandmother/etc. or aunt/great-aunt/etc. of Elsie. In the latter case, the reverse is true. So if $a > b$, we can simply swap the names of Elsie and Bessie, and swap $a$ and $b$, and we find ourselves in the $b > a$ case.

Now we can observe that either $a = 0$ or $a = 1$. In the former case, Elsie is the mother/grandmother/etc. of Bessie, whereas in the latter case she is the aunt/great-aunt/etc. of Bessie. So we simply output the appropriate number of "great-"s, depending on the value of $b$, possibly output "grand-", depending on $a$ and $b$, and then output either "mother" or "aunt", depending on $a$.

The overall algorithm is $O(N^3)$, since we call IS-ANCESTOR at most $N$ times. Since $N = 100$, this runs in the time limit, but it is possible to speed up the algorithm by not recomputing the ancestors of Elsie each time.

#include <iostream>
#include <algorithm>
#include <string>
#include <cassert>
using namespace std;
#define MAXN 100

int N;
string daughter[MAXN];
string mother[MAXN];

// returns mother of cow, or "" if mother does not exist
string find_mother(string cow)
{
for(int i=0;i<N;i++)
if(cow == daughter[i])
return mother[i];
return "";
}

// returns number of generations by which cow1 is removed from cow2
// if cow1 is a direct ancestor of cow2.
// returns -1 otherwise.
int is_ancestor(string cow1, string cow2)
{
int counter = 0;
while(cow2 != "")
{
if(cow1 == cow2)
return counter;
cow2 = find_mother(cow2);
counter++;
}
return -1;
}

int main()
{
string bessie, elsie;
cin >> N >> bessie >> elsie;
for(int i=0;i<N;i++)
cin >> mother[i] >> daughter[i];

string cow = bessie;
int b = 0;
while(cow != "")
{
if(is_ancestor(cow, elsie) != -1)
break;
cow = find_mother(cow);
b++;
}
if(cow == "")
{
cout << "NOT RELATED\n";
return 0;
}
int a = is_ancestor(cow, elsie);
if(a == 1 && b == 1) cout << "SIBLINGS\n";
else if(a > 1 && b > 1) cout << "COUSINS\n";
else
{
if(a > b) swap(elsie, bessie), swap(a, b);
cout << elsie << " is the ";
for(int i=0;i<b-2;i++) cout << "great-";
if(b > 1 && a == 0) cout << "grand-";
if(a == 0) cout << "mother";
else cout << "aunt";
cout << " of " << bessie << '\n';
}
return 0;
}

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