Output format error.是什么鬼啊

#include<iostream>
#include<cstdio>
#include<string>
#include<vector>
#include<algorithm>
#define bug puts("----------");
using namespace std;
const int maxn=25005;
int n;
int cnt;
string a,ans;
struct t{
    int nex[30];
    bool visit;
    int num,SIZE;
}tree[maxn];
vector<int> v[maxn];
void qinsert(string s){
    int p=0;
    for(int i=0;i<s.size();i++){
        int c=s[i]-'a';
        if(!tree[p].nex[c])
            tree[p].nex[c]=++cnt;
        p=tree[p].nex[c];   
        tree[p].num=c;
    }
    tree[p].visit=1;
}
void build(int node){
    for(int i=0;i<26;i++)
        if(tree[node].nex[i]){
            v[node].push_back(tree[node].nex[i]);
            build(tree[node].nex[i]);
        }
}
bool cmp(int X,int Y){
    return tree[X].SIZE<tree[Y].SIZE;
}
void dfs(int node){
    tree[node].SIZE++;
    for(int i=0;i<v[node].size();i++){
        dfs(v[node][i]);
//      cout<<node<<' '<<v[node][i]<<' '<<tree[v[node][i]].SIZE<<' '<<tree[node].SIZE<<endl;
//      cout<<tree[node].SIZE<<' '<<tree[v[node][i]].SIZE<<endl;
        tree[node].SIZE+=tree[v[node][i]].SIZE;
//      cout<<tree[node].SIZE<<' '<<tree[v[node][i]].SIZE<<endl;
//      cout<<node<<' '<<v[node][i]<<' '<<tree[v[node][i]].SIZE<<' '<<tree[node].SIZE<<endl;
    }
    sort(v[node].begin(),v[node].end(),cmp);
}
void dfs2(int node,int fa){
    ans+=char(tree[node].num+'a');
    if(tree[node].visit)
        ans+='P';
    for(int i=0;i<v[node].size();i++){
        dfs2(v[node][i],node);
        ans+='-';
    }
}
int main(){
    scanf("%d",&n);
    for(int i=1;i<=n;i++){
        cin>>a;
        qinsert(a);
    }
    build(0);
    dfs(0);
//  for(int i=0;i<=cnt;i++){
//      cout<<i<<endl;
//      for(int j=0;j<v[i].size();j++)
//          cout<<v[i][j]<<' ';
//      cout<<endl;
//  }
    dfs2(0,0);
    int len=ans.size()-1;
    while(ans[len]=='-')
        len--;
    cout<<len<<endl;
    for(int i=1;i<len;i++)
        cout<<char(ans[i])<<endl;
}