Bài "Shortest route" trên codechef?

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Đề bài: Shortest Route - Submit | CodeChef

Code của em:

#include<iostream>
#include<string>
#include<vector>
#include<algorithm>
#include<queue>
#include<string>

#define ll long long int

using namespace std;

ll lower_bound(vector<ll>& v, int x)
{
	ll l = 0, r = (ll)v.size() - 1;

	while (l <= r)
	{
		ll m = (l + r) / 2;

		if (v[m] <= x)  l = m + 1;
		else r = m - 1;
	}

	if (l - 1 != -1) return v[l - 1];

	return -1;
}

ll upper_bound(vector<ll>& v, int x)
{
	ll l = 0, r = (ll)v.size() - 1;

	while (l <= r)
	{
		ll m = (l + r) / 2;

		if (v[m] >= x)  r = m - 1;
		else l = m + 1;
	}

	if (r + 1 != (ll)v.size()) return v[r + 1];

	return -1;
}


int main()
{
	int num;

	ll n, m;

	cin >> num;

	for (int i = 0; i < num; i++)
	{
		cin >> n >> m;

		// 1: right, 2: left
		vector<ll> left_train, right_train, ans(m);

		for (int j = 0; j < n; j++)
		{
			int x;

			cin >> x;

			if (x == 1) right_train.push_back(j);
			else if (x == 2) left_train.push_back(j);
		}

		for (int j = 0; j < m; j++)
		{
			ll dest;
			cin >> dest;
			dest--;

			// find the greast <= dest that has right train
			// find the smallest >= dest that has left train

			ll i1 = lower_bound(right_train, dest);
			ll i2 = upper_bound(left_train, dest);

			if (i1 == -1 && i2 == -1) ans[j] = -1;
			else if (i1 == -1) ans[j] = i2 - dest;
			else if (i2 == -1) ans[j] = dest - i1;
			else ans[j] = min(dest - i1, i2 - dest);
		}

		for (int j = 0; j < m; j++) cout << ans[j] << " ";

		cout << endl;

		left_train.clear(); right_train.clear(); ans.clear();
	}

	return 0;
}

Bài này khá đơn giản nhưng không hiểu sao code của em vẫn bị WA. Giúp em với ạ!