USACO 2018 December Contest, Bronze Problem 3. Back and Forth

Debugging Help
#1

Problem

While solving the problem, I encountered a bug that caused the i value within the for loop to randomly become 1000, 1999, and other incorrect numbers. I was wondering where this originated from within the code, and how I could possibly fix it. Thanks!

#include <bits/stdc++.h>

using namespace std;

set<int> b1, b2, ans;

int c1[101] = {}, c2[101] = {};

void search(int m, int n) {
	if(n == 4) {
		ans.insert(m);
		return;
	}

	if(n % 2 == 0) {
		for(int i : b1) {
			cout<<i<<endl;
			c1[i]--;
			c2[i]++;
			if(c1[i]==0) {
				b1.erase(i);
			}
			b2.insert(i);
			search(m-i,n+1);
			c1[i]++;
			c2[i]--;
			if(c2[i]==0) {
				b2.erase(i);
			}
			b1.insert(i);
		}
	}

	else{ 
		for(int i : b2) {
			cout<<i<<endl;
			c2[i]--;
			c1[i]++;
			if(c2[i]==0) {
				b2.erase(i);
			}
			b1.insert(i);
			search(m+i,n+1);
			c2[i]++;
			c1[i]--;
			if(c1[i]==0) {
				b1.erase(i);
			}
			b2.insert(i);
		}
	}

}

int main() {  
	freopen("backforth.in","r",stdin);
	freopen("backforth.out","w",stdout);   

	for(int i = 0; i < 10; i++) {
		int t; cin>>t;
		b1.insert(t);
		c1[t]++;
	}

	for(int i = 0; i < 10; i++) {
		int t; cin>>t;
		b2.insert(t);
		c2[t]++;
	}

	search(1000, 0);

	cout<<(int) ans.size();
}
#2

Just to make sure, you’re talking about the for loops in the main method, correct?

#3

I meant the ones in the search loop, sorry

#4

Try putting more debug prints. The only way the for loop would “randomly” go through those outrageous values is if you put them in there in the first place.

#5

Here is updated code:

#include <bits/stdc++.h>

using namespace std;

set<int> b1, b2;
set<int> possible;
vector<int> a;

int c1[101] = {}, c2[101] = {};

void search(int m, int n) {
	cout<<m<<" "<<n<<endl;
	if(n == 4) {
		possible.insert(m);
		return;
	}

	if(n % 2 == 0) {
		for(int i : b1) {
			if(i>100) {
				cout<<i<<endl;
				return;
			}
			cout<<i<<endl;
			c1[i]--;
			c2[i]++;
			if(c1[i]==0) {
				b1.erase(i);
			}
			b2.insert(i);
			
			search(m-i,n+1);

			cout<<m<<" "<<n<<endl;
			c1[i]++;
			c2[i]--;
			if(c2[i]==0) {
				b2.erase(i);
			}
			b1.insert(i);
			
		}
	}

	else{ 
		for(int i : b2) {
			if(i>100) {
				cout<<i<<endl;
				return;
			}
			cout<<i<<endl;
			c2[i]--;
			c1[i]++;
			if(c2[i]==0) {
				b2.erase(i);
			}
			b1.insert(i);
			search(m+i,n+1);
			cout<<m<<" "<<n<<endl;
			c2[i]++;
			c1[i]--;
			if(c1[i]==0) {
				b1.erase(i);
			}
			b2.insert(i);
			
		}
	}

}

int main() {  
	freopen("backforth.in","r",stdin);
	freopen("backforth.out","w",stdout);   

	for(int i = 0; i < 10; i++) {
		int t; cin>>t;
		b1.insert(t);
		c1[t]++;
	}

	for(int i = 0; i < 10; i++) {
		int t; cin>>t;
		b2.insert(t);
		c2[t]++;
	}

	search(1000, 0);
}

What I realized was that for some reason, when I reached the end, or when n=4, the adding of the end value to the possible set somehow caused it to be added into the b1 set, and if I removed the possible set, the error would disappear. Is there some part of the set functionality that is causing this?