我是用链表做的这道题，用vector students保存每个学生的编号，以及左边和右边的同学的编号；用most_left记录开始位置。当左边或右边没有同学时值为-1。可是在实际运行时most_left的右边总会是-1，不知道是怎么回事

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
struct Student {
	int id;
	int left = -1;
	int right = -1;
	bool hidden = false;
};


int main() {
	int n;
	cin >> n;
	vector<Student> students;
	Student first;
	first.id = 1;
	students.push_back(first);
	Student most_left = students[0];
	for (int i = 0; i < n-1; ++i) {
		int k, p;
		scanf("%d%d", &k, &p);
		Student to_add;
		to_add.id = i+2;
		if (k == most_left.id && p == 0) {
			most_left = to_add;
		}
		if (p == 0) {
			to_add.right = k;
			to_add.left = students[k-1].left;
			students[students[k-1].left-1].right = to_add.id;
			students[k-1].left = to_add.id;
		} else if (p == 1) {
			to_add.left = k;
			to_add.right = students[k-1].right;
			students[students[k-1].right-1].left = to_add.id;
			students[k-1].right = to_add.id;
		}
		students.push_back(to_add);
		//cout << "Pushing back " << to_add.left << " " << to_add.id << " " << to_add.right << endl;
	}
	int M;
	cin >> M;
	for (int j = 0; j < M; ++j) {
		int id_to_remove;
		cin >> id_to_remove;
		students[id_to_remove-1].hidden = true;
	}
	while (most_left.right != -1) {
		if (!most_left.hidden) cout << most_left.id << " ";
		most_left = students[most_left.right-1];
	}
	cout << most_left.id << " " << most_left.right << endl;
}