【0】做题心得 - 2025 NOIP #67 - T1【贪心】

#include<bits/stdc++.h>
using namespace std;
const int N=1e6+10,V=26+5;
int n,ans;
char a[N];
stack<int>stk[V];
int solve(){
	for(int c=0;c<26;c++)
		while(stk[c].size()) stk[c].pop();
	for(int i=1;i<=n;i++)
		stk[a[i]-'a'].push(i);
	int L=n,R=0,res=0;
	for(int i=1;i<=n&&L>i;i++){
		int ch=stk[a[i]-'a'].top(); stk[a[i]-'a'].pop();
		if(L<=ch&&ch<=R) continue;
		if(ch==n-i+1) res+=max(R-L+1,0), R=n-i;
		else L=min(L,ch), R=max(R,n-i+1); 
	}
	res+=max(R-L+1,0);
	return res;
}
int main(){
	freopen("poem.in","r",stdin);
	freopen("poem.out","w",stdout);
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	cin>>n;
	for(int i=1;i<=n;i++)
		cin>>a[i];
	ans=min((int)1e9,solve());
	reverse(a+1,a+n+1);
	ans=min(ans,solve());
	cout<<min(ans,n-1);
	return 0;
}