Help me in solving REPLACEGAME problem

help
,
1

My issue

please help me

My code

#include <bits/stdc++.h>
using namespace std;

void solve(){
    int n,k;
    cin>>n>>k;
    string f,s;
    cin>>f>>s;
    vector<int>prev(n);
    for(int i=0;i<n;i++){
        prev[i]=i;
        if(i and s[i]==s[i-1]) prev[i]=prev[i-1];
    }
    vector<pair<int,char>>changes;
    int j=n;
    int pos_=n;
    char c_;
    for(int i=0;i<k;i++){
        j--;
        if(j-k+1<0) break;
        if(j>=pos_) f[j]=c_;
        if(s[j]==f[j]) continue;
        f[j]=s[j];
        pos_=j-k+1,c_=f[j];
        changes.push_back({pos_,c_});
        
    }
    while(--j>=pos_) f[j]=c_; 
    bool change[n]={false};
    char c;
    int pos=-1;
    for(int i=0;i<n;i++){
        char &curr=f[i];
        if(pos>=i) curr=c;
        if(curr==s[i]) continue;
        int p=prev[i];
        p=max(p,i-k+1);
        if(p+k-1>=n) continue;
        if(!change[p]) changes.push_back({p,s[i]});
        change[p]=true;
        pos=p+k-1,c=s[i];
        f[i]=s[i];
    }
    
    if(f!=s) cout<<-1<<endl;
    else{
        cout<<changes.size()<<endl;
        for(auto t:changes) cout<<t.first+1<<" "<<t.second<<endl;
    }
}

int main() {
	int t;
	cin>>t;
	while(t--) solve();
}

Problem Link: Replacing Game Practice Coding Problem