L2-004 这是二叉搜索树吗？

#include<iostream>
#include<vector>
using namespace std;
const int maxn = 1e3 + 10;
int pre[maxn], n;
bool is;
vector<int>ss;


void f(int l, int r){
    if (l > r)return;
    int tr = l + 1;
    int tl = r;
    if (!is){
        while (tr <= r&&pre[tr] < pre[l])tr++;
        while (tl > l&&pre[tl]>=pre[l])--tl;
    }
    else{
        while (tr <= r&&pre[tr] >= pre[l])tr++;
        while (tl > l&&pre[tl] < pre[l])--tl;
    }
    if (tr - tl != 1)return;

    f(l + 1, tl);        f(tr, r);
    ss.push_back(pre[l]);
}

int main(){
    cin >> n;
    for (int i = 0; i < n; ++i)cin >> pre[i];
    f(0, n - 1);

    if (ss.size() != n){
        is = 1;
        ss.clear();
        f(0, n - 1);
    }
    if (ss.size() != n)cout << "NO" << endl;
    else{
        cout << "YES" << endl;
        cout << ss[0];
        for (int i = 1; i < n; ++i)
            cout << " " << ss[i];
        cout << endl;
    }
}