Problem A :

#include <cstdio>
#include <cstring>
#include <cmath>
#include <iostream>
using namespace std;

int main(){
    int n,a[6],b[6];
    scanf("%d",&n);
    memset(a,0,sizeof(a));
    memset(b,0,sizeof(b));
    for(int i=0;i<n;i++){
        int t;
        scanf("%d",&t);
        a[t]++;
    }
    for(int i=0;i<n;i++){
        int t;
        scanf("%d",&t);
        b[t]++;
    }

    int res=0,f=0,c=0;

    for(int i=1;i<=5;i++){
        int t=a[i]-b[i];
        if(t%2!=0){
            f=1;
            break;
        }else{
            res += t/2;
            c += abs(t/2);
        }
    }

    if(f || res!=0) printf("-1");
    else printf("%d",c/2);

    printf("\n");
    return 0;
}

Problem B :

#include <cstdio>
#include <cstring>
#include <cmath>
#include <iostream>
using namespace std;

#define maxn 2000000005

int main(){
    string str;
    int k;
    cin>>str>>k;
    int len = str.length();
    if(str=="0"){
        printf("0\n");
        return 0;
    }
    int Z=0,N=0;
    for(int i=len-1;i>=0;i--){
        if(Z==k){
            break;
        }
        if(str[i]=='0') Z++;
        else N++;
    }
    if(Z==k) cout<<N<<endl;
    else printf("%d\n",len-1);

    return 0;
}

Problem C :

#include <cstdio>
#include <iostream>
#include <algorithm>
using namespace std;

const int maxn = 200005;

struct Node{
    int c,index;
};

bool check(Node a,Node b){
    return a.c>b.c;
}

int main(){
    int n,k,a[maxn],b[maxn];
    Node c[maxn];
    scanf("%d%d",&n,&k);
    for(int i=0;i<n;i++) scanf("%d",&a[i]);
    for(int i=0;i<n;i++) scanf("%d",&b[i]);

    for(int i=0;i<n;i++){
        c[i].c = b[i]-a[i];
        c[i].index = i;
    }

    sort(c, c+n, check);

    int res=0;

    for(int i=0;i<k;i++)
        res += a[c[i].index];
    for(int i=k;i<n;i++)
        if(c[i].c>0){
            res += a[c[i].index];
        }else{
            res += b[c[i].index];
        }

    cout<<res<<endl;

    return 0;
}

Problem D:

#include <cstdio>
#include <iostream>
#include <algorithm>
using namespace std;

#define F(i,a,b) for(int i=a;i<=b;i++)
#define D(i,a,b) for(int i=a;i<b;i++)


const int maxn = 200005;

string a,b;
int len,lenl, num[maxn],vis[maxn];


int check(int mid){
    F(i,1,len) vis[i]=0;
    F(i,mid+1,len) vis[num[i]]=1;
    int cnt = 0;
    D(i,0,len){
        if(!vis[i+1]) continue;
        if(a[i]==b[cnt]) cnt++;
        if(cnt>=lenl) return 1;
    }
    return 0;
}

int main(){
    while (cin>>a>>b) {
        len = (int) a.length();
        lenl = (int) b.length();
        F(i,1,len) scanf("%d",&num[i]);

        int l=0,r=len-1,mid,ans;
        while (l<=r) {
            mid = (l+r)>>1;
            check(mid)?l=mid+1,ans=mid : r=mid-1;
        }
        cout<<ans<<endl;
    }
}