Language: C++

#include <bits/stdc++.h>
using namespace std;
#define INF 1<<30
#define endl '\n'
#define maxn 100005
#define tc printf("Case %d: ", cs)
#define tcn printf("Case %d:\n", cs);
#define FASTIO ios_base::sync_with_stdio(false), cin.tie(0), cout.tie(0);
typedef long long ll;
const double PI = acos(-1.0);

#define dbg1(x) cerr << #x << " = " << x << endl;
#define dbg2(x, y) cerr << #x << " = " << x << ", " << #y << " = " << y << endl;
#define dbg3(x, y, z) cerr << #x << " = " << x << ", " << #y << " = " << y << ", " << #z << " = " << z << endl;
#define dbg4(w,x, y, z) cerr << #w << " = " << w << ", " <<#x << " = " << x << ", " << #y << " = " << y << ", " << #z << " = " << z << endl;

template < typename F, typename S >
ostream& operator << ( ostream& os, const pair< F, S > & p ) {
	return os << "(" << p.first << ", " << p.second << ")";
}

template < typename T >
ostream &operator << ( ostream & os, const vector< T > &v ) {
	os << "{";
	for (auto it = v.begin(); it != v.end(); ++it) {
		if ( it != v.begin() ) os << ", ";
		os << *it;
	}
	return os << "}";
}

template < typename T >
ostream &operator << ( ostream & os, const set< T > &v ) {
	os << "[";
	for (auto it = v.begin(); it != v.end(); ++it) {
		if ( it != v.begin()) os << ", ";
		os << *it;
	}
	return os << "]";
}

template < typename F, typename S >
ostream &operator << ( ostream & os, const map< F, S > &v ) {
	os << "[";
	for (auto it = v.begin(); it != v.end(); ++it) {
		if ( it != v.begin() ) os << ", ";
		os << it -> first << " = " << it -> second ;
	}
	return os << "]";
}

#define dbg(args...) do {cerr << #args << " : "; faltu(args); } while(0)

clock_t tStart = clock();
#define timeStamp dbg("Execution Time: ", (double)(clock() - tStart)/CLOCKS_PER_SEC)

void faltu () { cerr << endl; }

template <typename T>
void faltu( T a[], int n ) {
	for (int i = 0; i < n; ++i) cerr << a[i] << ' ';
	cerr << endl;
}

template <typename T, typename ... hello>
void faltu( T arg, const hello &... rest) { cerr << arg << ' '; faltu(rest...); }
// Program showing a policy-based data structure.
#include <ext/pb_ds/assoc_container.hpp> // Common file 
#include <ext/pb_ds/tree_policy.hpp>
#include <functional> // for less 
#include <iostream>
using namespace __gnu_pbds;
using namespace std;
// GNU link : https://goo.gl/WVDL6g
typedef tree<int, null_type, less_equal<int>, rb_tree_tag,
        tree_order_statistics_node_update>
        new_data_set;

/**___________________________________________________**/

const int N = 1e5 + 5;
int clr = 0;


void colorEdges(int ptr, vector<vector<pair<int, int> > >& gra,
                vector<int>& edgeColors, bool isVisited[])
{
	queue<int> q;
	int c = 0;

	set<int> colored;

	// return if isVisited[ptr] is true
	if (isVisited[ptr])
		return;

	// Mark the current node visited
	isVisited[ptr] = 1;

	// Traverse all edges of current vertex
	for (int i = 0; i < (int) gra[ptr].size(); i++) {
		// if already colored, insert it into the set
		if (edgeColors[gra[ptr][i].second] != -1)
			colored.insert(edgeColors[gra[ptr][i].second]);
	}

	for (int i = 0; i < (int) gra[ptr].size(); i++) {
		// if not visited, inset into the queue
		if (!isVisited[gra[ptr][i].first])
			q.push(gra[ptr][i].first);

		if (edgeColors[gra[ptr][i].second] == -1) {
			// if col vector -> negative
			while (colored.find(c) != colored.end())

				// increment the color
				c++;

			// copy it in the vector
			edgeColors[gra[ptr][i].second] = c;

			// then add it to the set
			colored.insert(c);
			c++;
			clr = max(clr, c);
		}
	}

	// while queue's not empty
	while (!q.empty()) {
		int temp = q.front();
		q.pop();

		colorEdges(temp, gra, edgeColors, isVisited);
	}

	return;
}

int main()
{
#ifndef ONLINE_JUDGE
	freopen("in.txt", "r", stdin);
	freopen("out.txt", "w", stdout);
	freopen("error.txt", "w", stderr);
#endif
	vector<vector<pair<int, int> > > graph;

	vector<int> edgeColors;

	bool isVisited[N] = { 0 };
	int n;
	cin >> n;
	cin >> n;

	graph.resize(n);
	edgeColors.resize(n - 1, -1);

	for (int i = 1; i < n ; i++) {
		int u, v;
		cin >> u >> v;
		--u;
		--v;
		graph[u].push_back(make_pair(v, i));
		graph[v].push_back(make_pair(u, i));
	}

	colorEdges(0, gra, edgeColors, isVisited);

	cout << clr << endl;
	for (int i = 0; i < edge; i++)
		cout << edgeColors[i] + 1 << "\n";

	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:168:23: error: 'gra' was not declared in this scope; did you mean 'graph'?
  168 |         colorEdges(0, gra, edgeColors, isVisited);
      |                       ^~~
      |                       graph
a.cc:171:29: error: 'edge' was not declared in this scope
  171 |         for (int i = 0; i < edge; i++)
      |                             ^~~~