When codeforces first opened, how did it get so popular?

I am having difficulty solving a problem on a different coding website. The problem goes like this:

The m × n rectangular grid is a graph whose vertices correspond to the points in the plane with x-coordinates being integers in the range 0, … , n-1 and y-coordinates being integers in the range 0, … , m-1, and two vertices are joined by an edge whenever the corresponding points are at unit distance apart. For example, a 4 × 6 rectangular grid is shown in Figure 1. The grid has n vertices appearing in each of m rows and m vertices in each of n columns. The vertex in row i and column j is denoted by (i, j), where 0 ≤ i ≤ m — 1 and 0 ≤ j ≤ n — 1.

If we add an edge joining two vertices (i, 0) and (i, n-1) of the m × n rectangular grid for every row i ∈ {0, … , m-1} and moreover, add an edge between two vertices (0, j) and (m-1, j) for every column j ∈ {0, … , n-1}, then each row forms a cycle of length n and each column forms a cycle of length m, as illustrated in Figure 2. The resulting graph is often called an m × n toroidal grid, because it can be drawn on a torus without edge crossings.

Given an m × n toroidal grid, you are to write a program to find a cycle that visits every vertex exactly once. Here, the required cycle may be represented as a sequence, (v1, v2, … , vmn), of mn distinct vertices of the graph such that vk and vk+1 are adjacent for all k ∈ {1, … , mn-1} and moreover, vmn and v1 are adjacent.

I got a compile error my code:

#include<stdio.h>

int main() {

	int t;
	scanf("%d", &t);

	while(t--){

		int r, c;

		scanf("%d %d", &r, &c);

		printf("1\n");

		for(int i=0; i=1; j--){
					printf("(%d,%d)\n", i, j);
				}
			}else{
				for(int j=1; j=1; i--){
			printf("(%d,0)\n", i, 0);
		}

	}

}

What was wrong?

Not trying to hustle here, but how and what does it mean to hack during a contest? Are you supposed to find a bug in a different person's code?

Is it often for the Judge Queue to go wrong?

I'd like to know if I'm allowed to get some hints on problems(previous ones from past contest). If possible, can I have some help on the problem:Sonya and Queries?

I'm afraid oyu8201 is cheating, his code:#include <bits/stdc++.h>

define fr(a, init, end) for(int a = init; a < end; ++a)

define add_wedge(a, b, c) to[z] = b; ant[z] = adj[a]; adj[a] = z; cost[z] = c; z++

define add_edge(a, b) to[z] = b; ant[z] = adj[a]; adj[a] = z++

define fre(i, u) for(int i = adj[u]; i != -1; i = ant[i])

define sc(a) scanf("%d", &a)

define debug(x) cerr << ">>> " << #x << " = " << x << endl;

define dbg(x) if(1) cerr << ">>> " << x << endl;

define _ << " , " <<

define endl '\n'

using namespace std;

typedef long long ll; typedef pair<int, int> ii;

const int N = int(1e6);

int col[N]; int to[N], ant[N], adj[N], z;

void dfs(int u) { fre(i, u) { if(col[to[i]] == -1) { col[to[i]] = col[u] ^ 1; dfs(to[i]); } } } int main() { ios::sync_with_stdio(false);cin.tie(NULL);

memset(col, -1, sizeof col);
memset(adj, -1, sizeof adj);
z = 0;

int n;
cin >> n;

int a, b;
vector<ii> p;

fr(i, 0, n)
{
    cin >> a >> b;
    --a, --b;
    add_edge(a, b);
    add_edge(b, a);
    p.push_back({a, b});
}

fr(i, 0, 2 * n)
{
    add_edge(i, i + 1);
    add_edge(i + 1, i);
    ++i;
}

fr(i, 0, 2 * n)
{
    if(col[i] == -1)
    {
        col[i] = 0;
        dfs(i);
    }
}

fr(i, 0, n)
{
    cout << col[p[i].first] + 1 << ' ' << col[p[i].second] + 1 << endl;
}

return 0;

}

my code: #include <bits/stdc++.h>

define fr(a, init, end) for(int a = init; a < end; ++a)

define add_wedge(a, b, c) to[z] = b; ant[z] = adj[a]; adj[a] = z; cost[z] = c; z++

define add_edge(a, b) to[z] = b; ant[z] = adj[a]; adj[a] = z++

define fre(i, u) for(int i = adj[u]; i != -1; i = ant[i])

define sc(a) scanf("%d", &a)

define debug(x) cerr << ">>> " << #x << " = " << x << endl;

define dbg(x) if(1) cerr << ">>> " << x << endl;

define _ << " , " <<

define endl '\n'

using namespace std;

typedef long long ll; typedef pair<int, int> ii;

const int N = int(1e6);

int col[N]; int to[N], ant[N], adj[N], z;

void dfs(int u) { fre(i, u) { if(col[to[i]] == -1) { col[to[i]] = col[u] ^ 1; dfs(to[i]); } } } int main() { ios::sync_with_stdio(false);cin.tie(NULL);

memset(col, -1, sizeof col);
memset(adj, -1, sizeof adj);
z = 0;

int n;
cin >> n;

int a, b;
vector<ii> p;

fr(i, 0, n)
{
    cin >> a >> b;
    --a, --b;
    add_edge(a, b);
    add_edge(b, a);
    p.push_back({a, b});
}

fr(i, 0, 2 * n)
{
    add_edge(i, i + 1);
    add_edge(i + 1, i);
    ++i;
}

fr(i, 0, 2 * n)
{
    if(col[i] == -1)
    {
        col[i] = 0;
        dfs(i);
    }
}

fr(i, 0, n)
{
    cout << col[p[i].first] + 1 << ' ' << col[p[i].second] + 1 << endl;
}

return 0;

}