题解 | Problem D

#include <iostream>
using namespace std;

void dfs(string pre, string in) {
    if (pre.empty()) return;
    char root = pre[0];
    int k = in.find(root);
    dfs(pre.substr(1, k), in.substr(0, k));
    dfs(pre.substr(k + 1), in.substr(k + 1));
    cout << root;
}
int main() {
    string pre, in;
    while (cin >> pre >> in) {
        dfs(pre, in);
        cout << endl;
    }
}

树生成的固定代码