#include <cstdio>

struct Node {
    int left, right;
    Node() { this->left = this->right = 0; }
}tree[30];

void pre_order(int u) {
    putchar(u + 'a' - 1);
    if (tree[u].left) pre_order(tree[u].left);
    if (tree[u].right) pre_order(tree[u].right);
}

int main() {
    int n;
    scanf("%d\n", &n);
    int root = 0;
    while (n--) {
        // printf("%d\n", n);
        char a, b, c;
        scanf("%c%c%c\n", &a, &b, &c);
        //printf("%c %c %c\n", a, b, c);
        if (!root) root = a - 'a' + 1;
        if (b != '*') tree[a-'a'+1].left = b-'a'+1;
        if (c != '*') tree[a-'a'+1].right = c-'a'+1;
    }
    pre_order(root);
    putchar('\n');
    return 0;
}

为什么本地命令行输入还要Ctrl+Z？

？？？