題意：給你n個數，和m，讓我們從1-m選數字，使得n個數奇數和偶數個數相同切各不相同

思路：模擬

 1 #include<bits/stdc++.h>
 2 using namespace std;
 3 typedef long long ll;
 4 
 5 int n,m;
 6 set<ll > s;
 7 map<ll ,int > ma;
 8 ll b[200005];
 9 ll a[200005];
10 int main(){
11     cin>>n>>m;
12    ll x;
13     for(int i=1;i<=n;i++){
 
14         scanf("%lld",&x);
15         a[i]=x;
16         s.insert(x);
17     }
18     int s1=0,s2=0;
19     for(set<ll >::iterator it=s.begin();it!=s.end();it++){
20         ma[*it]++;
21         if(*it&1) s1++;
22         else s2++;
23     }
24     s1=n/2-s1;
25     s2=n/2-s2;
26     if(s1<0
 
) s1=0;
27     if(s2<0) s2=0;
28     int sum=0;
29     int l=0;
30     for(int i=1;i<=m;i+=2){
31         if(s1==0) break;
32         if(!ma[i]){
33             s1--;
34             b[++l]=i;
35             sum++;
36         }
37 
38     }
39     for(int i=2;i<=m;i+=2){
40         if(s2==0) break;
41
 
         if(!ma[i]){
42            b[++l]=i;
43             s2--;sum++;
44         }
45 
46     }
47 
48     if(s1||s2) cout<<-1<<endl;
49     else {
50         int r=1;
51         int sum1=0,sum2=0;
52         cout<<sum<<endl;
53         for(int i=1;i<=n;i++){
54             int t=0;
55             if(ma[a[i]]){
56                 if(a[i]&1){
57                     if(sum1<n/2) {
58                         printf("%d ",a[i]);
59                         sum1++;ma[a[i]]=0;t=1;
60                     }
61                 }
62                 else {
63                     if(sum2<n/2){
64                         printf("%d ",a[i]);
65                         sum2++;ma[a[i]]=0;t=1;
66                     }
67                 }
68             }
69             if(!t) printf("%d ",b[r++]);
70         }
71         printf("\n");
72     }
73 }

Codeforces Round #386 (Div. 2) E - Numbers Exchange