748B - Santa Claus and Keyboard Check - CodeForces Solution

#include<bits/stdc++.h>
#define ll long long 
using namespace std;
const ll INF=0x3f3f3f3f;
const ll N=1e6+10,M=2e6+10;
ll a[N],x[N],y[N];
vector<char>ans;
void solve(){
	string s1,s2;
	cin>>s1>>s2;
	ll len=s1.size();
	for(ll i=0;i<len;i++){
		if(a[s1[i]]!=0 && a[s1[i]]!=s2[i]){
			cout<<-1<<endl;
			return;
		}
		if(a[s2[i]]!=0 && a[s2[i]]!=s1[i]){
			cout<<-1<<endl;
			return;
		}
		if(a[s1[i]]==s2[i])continue;
		if(a[s2[i]]==s1[i])continue;
		a[s1[i]]=s2[i];
		a[s2[i]]=s1[i];
		if(s1[i]==s2[i])continue;
		ans.push_back(s1[i]);
		ans.push_back(s2[i]);
	}
	cout<<ans.size()/2<<endl;
	for(ll i=0;i<ans.size();i+=2){
		cout<<ans[i]<<" "<<ans[i+1]<<endl;
	}
} 
signed main(){ 
	int _=1;
//	cin>>_;
	while(_--)solve();
    return 0;
}
/*
he
ei
*/