第五届上海理工大学程序设计全国挑战赛-I-In Falsus · znzryb
## 思路讲解
那么容易想到，原来出现过的 $a???a??\rightarrow aaaaa??
$ 并不会增加。
$a?a?????c??c?→aaa?????cccc?$。
```cpp
for(int i=0;i<M;++i){
char ch=i+'a';
vector<ll> pos;
for(int j=1;j<=N;++j){
if(S[j]==ch){
pos.push_back(j);
}
}
if(pos.size()>=2){
for(int i=pos[0];i<=pos.back();++i){
if(S[i]=='?'){
S[i]=ch;
}
}
}
}
```
那么继续，我们把拆贡献的思路拓展到极致，就是说其实无论怎么样搞，搞完前面这套操作，还要继续操作的话，必定会增加1个代价，那么我们只要构造增加一个代价的字符串就行了。
用代码描述就是这样。
```cpp
FOR(i,1,N){
if(S[i]=='?'){
if(chs.empty()){
if(S[i-1]>='a' && S[i-1]<='z'){
S[i]=S[i-1];
}
}else{
S[i]=chs.back();chs.pop_back();
}
}
}
ROF(i,N,1){
if(S[i]=='?'){
if(chs.empty()){
if(S[i+1]>='a' && S[i+1]<='z'){
S[i]=S[i+1];
}
}else{
S[i]=chs.back();chs.pop_back();
}
}
}
```
## AC代码
[https://ac.nowcoder.com/acm/contest/view-submission?submissionId=78079278](https://ac.nowcoder.com/acm/contest/view-submission?submissionId=78079278)
::: 源代码
```cpp
// Problem: In Falsus
// Contest: NowCoder
// URL: https://ac.nowcoder.com/acm/contest/111679/I
// Memory Limit: 2048 MB
// Time Limit: 2000 ms
//
// Powered by CP Editor (https://cpeditor.org)
#include <bits/stdc++.h>
#define all(vec) vec.begin(),vec.end()
#define pb push_back
#define SZ(a) ((int) a.size())
#define FOR(i, a, b) for (int i = (a); i <= (b); ++i)
#define ROF(i, a, b) for (int i = (a); i >= (b); --i)
#define debug(var) cerr << #var <<":"<<var<<"\n";
#define lson(var) (var<<1)
#define rson(var) ((var<<1)+1)
using namespace std;
using ll = long long;using ull = unsigned long long;
using DB=double;using LD=long double;
// using i128 = __int128;
using pdd = pair<DB,DB>;using plb = pair<ll,bool>;
using pll = pair<ll,ll>;
using arr3 = array<ll,3> ;using arr2 = array<ll,2>;
constexpr ll MAXN=static_cast<ll>(1e6)+10,INF=static_cast<ll>(1e17)+9; // 1e18+9也是素数
constexpr ll mod=static_cast<ll>(1e9)+7;
constexpr double eps=1e-8;const double pi=acos(-1.0);
/*
*/
char S[MAXN];
bitset<30> vis;
inline void solve(){
ll N,M;cin>>N>>M;vis.reset();
S[0]='0';S[N+1]='0';
for(int i=1;i<=N;++i){
cin>>S[i];
}
for(int i=0;i<M;++i){
char ch=char(i+'a');
vector<ll> pos;
for(int j=1;j<=N;++j){
if(S[j]==ch){
pos.push_back(j);
}
}
if(pos.size()>=2){
for(int j=pos[0];j<=pos.back();++j){
if(S[j]=='?'){
S[j]=ch;
}
}
vis[i]=true;
}else if(pos.size()==1){
vis[i]=true;
}
}
vector<char> chs;
FOR(i,0,M-1){
if(vis[i]) continue;
char ch=char(i+'a');
chs.pb(ch);
}
FOR(i,1,N){
if(S[i]=='?'){
if(chs.empty()){
if(S[i-1]>='a' && S[i-1]<='z'){
S[i]=S[i-1];
}
}else{
S[i]=chs.back();chs.pop_back();
}
}
}
ROF(i,N,1){
if(S[i]=='?'){
if(chs.empty()){
if(S[i+1]>='a' && S[i+1]<='z'){
S[i]=S[i+1];
}
}else{
S[i]=chs.back();chs.pop_back();
}
}
}
ll ans=0;
for(int i=0;i<M;++i){
char ch=char(i+'a');
vector<ll> pos;
for(int j=1;j<=N;++j){
if(S[j]==ch){
pos.push_back(j);
}
}
if(pos.size()>=2){
ans+=(pos.back()-pos[0]);
}
}
cout<<ans<<"\n";
FOR(i,1,N){cout<<S[i];}
cout<<"\n";
}
int main()
{
ios::sync_with_stdio(false);
cin.tie(0);cout.tie(0);
ll lT;
cin>>lT;
while(lT--){
solve();
}
// solve();
return 0;
}
/*
AC
https://ac.nowcoder.com/acm/contest/view-submission?submissionId=78079278
*/
```
:::
## 心路历程（WA，TLE，MLE……）