(Analysis by Jonathan Paulson, Benjamin Qi)

This is a greedy problem. There's not an obvious brute force to try; there are too many ways to flip different substrings in different orders. But if you just play around with a few examples, it's pretty easy to solve them by hand. To solve the problem, you need to guess a rule for how to solve them, convince yourself that it's right, and then code it. Greedy problems can be dangerous, because it can be easy to convince yourself something is right even if it actually doesn't work. The reward is that they're usually easier to code.

In this case, I first simplified the problem by observing that it doesn't matter what order you flip substrings in. All that matters is how many times each cow gets flipped. So let's assume we do the flips from left to right (sorted by the first cow they flip).

Now imagine scanning through the string from left to right. Whenever you find a mismatch, you have to fix it right now (because we just said we won't go backwards). So we definitely have to start a flip at this cow. Where should we end the flip? We should definitely keep going as long as there's mismatches, since it's free to fix these in the same flip. But once we get to a currently-matching cow, we should stop. Why? Because if we flip that cow, we'll immediately need to flip it again on the very next move. And any useful flip we wanted to do as part of this move, we could've just done as part of that move. So it never saves us any moves to keep going.

This gives us a pretty simple solution; just flip all the ranges of mismatching cows. We can count how many ranges there are with one pass through the strings - just count the number of positions that *start* a mismatch.

Video Solution

C++ code (from Jonathan Paulson):

#include <iostream>
#include <string>
using namespace std;
using ll = long long;

int main() {
  freopen("breedflip.in", "r", stdin);
  freopen("breedflip.out", "w", stdout);
  ll n;
  cin >> n;
  string A;
  string B;
  cin >> A >> B;
  ll ans = 0;
  bool mismatched = false;
  for(ll i=0; i<n; i++) {
    if(A[i] != B[i]) {
      if(!mismatched) {
        mismatched = true;
    } else {
      mismatched = false;
  cout << ans << endl;

Java code (from Nick Wu). The Java code follows a different rule than the one described above. It works just as well though. See if you can work out why.

import java.io.*;
import java.util.*;
public class Solution {
  public static void main(String[] args) throws IOException {
    BufferedReader br = new BufferedReader(new FileReader("breedflip.in"));
    PrintWriter pw = new PrintWriter(new BufferedWriter(new FileWriter("breedflip.out")));
    int n = Integer.parseInt(br.readLine());
    char[] a = br.readLine().toCharArray();
    char[] b = br.readLine().toCharArray();
    int ret = 0;
    while(!new String(a).equals(new String(b))) {
      int lhs = 0;
      while(a[lhs] == b[lhs]) lhs++;
      int rhs = n-1;
      while(a[rhs] == b[rhs]) rhs--;
      for(int i = lhs; i <= rhs; i++) {
        if(a[i] == 'G') a[i] = 'H';
        else a[i] = 'G';

Another way to view the problem:

Place additional H's before and after both strings. Now define

$$A_{dif}[i]=\begin{cases} 1 & A[i] \neq A[i+1] \\ 0 & \text{otherwise} \\ \end{cases}$$
and define $B_{dif}$ similarly. We want to convert $B_{dif}$ to $A_{dif}$, and each modification to $B$ changes exactly two elements of $B_{dif}$. Thus, the answer is just the number of indices $i$ such that $A_{dif}[i]\neq B_{dif}[i]$ divided by two (as the numbers of ones in $A_{dif}$ and $B_{dif}$ are always even, this is guaranteed to be an integer).