题解 100% AC

#include <bits/stdc++.h>
using namespace std;
int main(){
    int n,m,j=-1;cin>>n;
    string a;cin>>a;
    cin>>m;
    bool flag=0;
    for(int i=0;i<a.length();i++){
    	if(a[i]=='.') flag=1;
        if(flag){
            j++;
            if(j<m-1) cout<<a[i];
            else{
            	if(j==m){
            		if((a[i+1]-'0')>=5&&a[i]!='9'){
                    	cout<<(a[i]-'0')+1;
                    	return 0;
                	}else if((a[i+1]-'0')>=5&&a[i]=='9'){
                		cout<<0;
                		return 0;
					}else{
                    	cout<<a[i];
                    	return 0;
                	}
				}else{
					if(a[i+1]=='9'&&(a[i+2]-'0')>=5) cout<<(a[i]-'0')+1;
					else cout<<a[i];
				}
            }
        }else cout<<a[i];
    }
	return 0;
}