CodeForces - 752B

#include<iostream>
#include<cstdio>
#include<queue>
#include<algorithm>
#include<cmath>
#include<ctime>
#include<set>
#include<map>
#include<stack>
#include<cstring>
#define inf 2147483647
#define ls rt<<1
#define rs rt<<1|1
#define lson ls,nl,mid,l,r
#define rson rs,mid+1,nr,l,r
#define N 100010
#define For(i,a,b) for(int i=a;i<=b;i++)
#define p(a) putchar(a)
#define g() getchar()

using namespace std;
char c1[N],c2[N];
int n,a[N],cnt;
bool b[N];
void in(int &x){
    int y=1;
    char c=g();x=0;
    while(c<‘0‘||c>‘9‘){
        if(c==‘-‘)y=-1;
        c=g();
    }
    while(c<=‘9‘&&c>=‘0‘){
        x=(x<<1)+(x<<3)+c-‘0‘;c=g();
    }
    x*=y;
}
void o(int x){
    if(x<0){
        p(‘-‘);
        x=-x;
    }
    if(x>9)o(x/10);
    p(x%10+‘0‘);
}
int main(){
    cin>>(c1+1)>>(c2+1);
    n=strlen(c1+1);
    For(i,1,n){
        if(c1[i]==c2[i]&&!a[(int)c1[i]]){
            a[(int)c1[i]]=(int)c1[i];
        }
        else
            if(c1[i]!=c2[i]&&!a[(int)c1[i]]&&!a[(int)c2[i]]){
                a[(int)c1[i]]=(int)c2[i];
                a[(int)c2[i]]=(int)c1[i];
                cnt++;
            }
            else
                if(c1[i]!=c2[i]&&(a[(int)c1[i]]!=(int)c2[i]||a[(int)c2[i]]!=(int)c1[i])){
                    o(-1);
                    return 0;
                }
                else
                    if(c1[i]==c2[i]&&a[(int)c1[i]]!=(int)c1[i]){
                        o(-1);
                        return 0;
                    }
    }
    o(cnt);p(‘\n‘);
    For(i,1,n){
        if(c1[i]!=c2[i]&&!b[c1[i]]){
            p(c1[i]);p(‘ ‘);p(c2[i]);p(‘\n‘);
            b[c1[i]]=b[c2[i]]=1;
        }
    }
    return 0;
}