8个MLE和2个WA，求调

#include<bits/stdc++.h>
using namespace std;
char l[100005];
char r[100005];
vector<char> a;
void preorder(char c)
{
    a.push_back(c);
    if(l['c'-60]!='*') preorder(l['c'-60]);
    if(r['c'-60]!='*') preorder(r['c'-60]);
}
int main()
{
    int n;
    cin>>n;
    for(int i=0;i<=n-1;i++)
    {
        string s;
        cin>>s;
        l[s[0]-60]=s[1];
        r[s[0]-60]=s[2];
    }
    preorder(1);
    for(int i=0;i<=a.size()-1;i++)
    {
        cout<<a[i];
    }
    return 0;
}