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Color tree with minimum colors such that colors of edges incident to a vertex are different

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  • Difficulty Level : Expert
  • Last Updated : 23 Feb, 2023
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Given a tree with N nodes. The task is to color the tree with the minimum number of colors(K) such that the colors of the edges incident to a vertex are different. Print K in first-line and then in next line print N – 1 space-separated integer represents the colors of the edges. Examples:

Input: N = 3, edges[][] = {{0, 1}, {1, 2}}
                   0
                  /  
                 1
                /
               2  
Output:
2
1 2
                   0
                  / (1) 
                 1
                / (2)
               2

Input: N = 8, edges[][] = {{0, 1}, {1, 2}, 
                           {1, 3}, {1, 4}, 
                           {3, 6}, {4, 5}, 
                           {5, 7}}
                    0
                   /
                  1
                / \ \
               2   3 4
                  /   \
                 6     5
                        \
                         7
Output:
4
1 2 3 4 1 1 2

Approach: First, let’s think about the minimum number of colors K. For every vertex v, deg(v) ≤ K should meet (where deg(v) denotes the degree of vertex v). In fact, there exists a vertex with all K different colors. First, choose a vertex and let it be the root, thus T will be a rooted tree. Perform a breadth-first search from the root. For each vertex, determine the colors of edges between its children one by one. For each edge, use the color with the minimum index among those which are not used as colors of edges whose one of endpoints is the current vertex. Then each index of color does not exceed K. Below is the implementation of the above approach: 

CPP




// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
 
// Add an edge between the vertexes
void add_edge(vector<vector<int> >& gr, int x,
              int y, vector<pair<int, int> >& edges)
{
    gr[x].push_back(y);
    gr[y].push_back(x);
    edges.push_back({ x, y });
}
 
// Function to color the tree with minimum
// number of colors such that the colors of
// the edges incident to a vertex are different
int color_tree(int n, vector<vector<int> >& gr,
               vector<pair<int, int> >& edges)
{
 
    // To store the minimum colors
    int K = 0;
 
    // To store color of the edges
    map<pair<int, int>, int> color;
 
    // Color of edge between its parent
    vector<int> cs(n, 0);
 
    // To check if the vertex is
    // visited or not
    vector<int> used(n, 0);
 
    queue<int> que;
    used[0] = 1;
    que.emplace(0);
 
    while (!que.empty()) {
 
        // Take first element of the queue
        int v = que.front();
        que.pop();
 
        // Take the possible value of K
        if (K < (int)gr[v].size())
            K = gr[v].size();
 
        // Current color
        int cur = 1;
 
        for (int u : gr[v]) {
 
            // If vertex is already visited
            if (used[u])
                continue;
 
            // If the color is similar
            // to it's parent
            if (cur == cs[v])
                cur++;
 
            // Assign the color
            cs[u] = color[make_pair(u, v)]
                = color[make_pair(v, u)] = cur++;
 
            // Mark it visited
            used[u] = 1;
 
            // Push into the queue
            que.emplace(u);
        }
    }
 
    // Print the minimum required colors
    cout << K << endl;
 
    // Print the edge colors
    for (auto p : edges)
        cout << color[p] << " ";
}
 
// Driver code
int main()
{
    int n = 8;
 
    vector<vector<int> > gr(n);
    vector<pair<int, int> > edges;
 
    // Add edges
    add_edge(gr, 0, 1, edges);
    add_edge(gr, 1, 2, edges);
    add_edge(gr, 1, 3, edges);
    add_edge(gr, 1, 4, edges);
    add_edge(gr, 3, 6, edges);
    add_edge(gr, 4, 5, edges);
    add_edge(gr, 5, 7, edges);
 
    // Function call
    color_tree(n, gr, edges);
 
    return 0;
}

Java




//Java program for the above approach
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Queue;
import java.util.TreeMap;
 
public class Main {
 
    // Add an edge between the vertexes
    public static void addEdge(List<List<Integer>> gr, int x, int y, List<int[]> edges) {
        gr.get(x).add(y);
        gr.get(y).add(x);
        edges.add(new int[] { x, y });
    }
 
    // Function to color the tree with minimum
    // number of colors such that the colors of
    // the edges incident to a vertex are different
    public static void colorTree(int n, List<List<Integer>> gr, List<int[]> edges) {
        // To store the minimum colors
        int K = 0;
 
        // To store color of the edges
        Map<int[], Integer> color = new TreeMap<>((a, b) -> a[0] != b[0] ? a[0] - b[0] : a[1] - b[1]);
 
        // Color of edge between its parent
        int[] cs = new int[n];
 
        // To check if the vertex is
        // visited or not
        boolean[] used = new boolean[n];
 
        Queue<Integer> que = new LinkedList<>();
        used[0] = true;
        que.offer(0);
 
        while (!que.isEmpty()) {
            // Take first element of the queue
            int v = que.poll();
 
            // Take the possible value of K
            if (K < gr.get(v).size()) {
                K = gr.get(v).size();
            }
 
            // Current color
            int cur = 1;
 
            for (int u : gr.get(v)) {
                // If vertex is already visited
                if (used[u]) {
                    continue;
                }
 
                // If the color is similar
                // to it's parent
                if (cur == cs[v]) {
                    cur++;
                }
 
                // Assign the color
                cs[u] = color.put(new int[] { u, v }, cur) == null ? cur : color.get(new int[] { u, v });
                color.put(new int[] { v, u }, cs[u]);
                cur++;
 
                // Mark it visited
                used[u] = true;
 
                // Push into the queue
                que.offer(u);
            }
        }
 
        // Print the minimum required colors
        System.out.println(K);
 
        // Print the edge colors
        for (int[] p : edges) {
            System.out.print(color.get(p) + " ");
        }
    }
 
    // Driver code
    public static void main(String[] args) {
        int n = 8;
 
        List<List<Integer>> gr = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            gr.add(new ArrayList<>());
        }
        List<int[]> edges = new ArrayList<>();
 
                // Add edges
        addEdge(gr, 0, 1, edges);
        addEdge(gr, 1, 2, edges);
        addEdge(gr, 1, 3, edges);
        addEdge(gr, 1, 4, edges);
        addEdge(gr, 3, 6, edges);
        addEdge(gr, 4, 5, edges);
        addEdge(gr, 5, 7, edges);
 
        // Function call
        colorTree(n, gr, edges);
    }
}
//This code is contributed by Potta Lokesh

Python




# Python3 implementation of the approach
from collections import deque as queue
 
gr = [[] for i in range(100)]
edges = []
 
# Add an edge between the vertexes
def add_edge(x, y):
    gr[x].append(y)
    gr[y].append(x)
    edges.append([x, y])
 
# Function to color the tree with minimum
# number of colors such that the colors of
# the edges incident to a vertex are different
def color_tree(n):
 
    # To store the minimum colors
    K = 0
 
    # To store color of the edges
    color = dict()
 
    # Color of edge between its parent
    cs = [0] * (n)
 
    # To check if the vertex is
    # visited or not
    used = [0] * (n)
 
    que = queue()
    used[0] = 1
    que.append(0)
 
    while (len(que) > 0):
 
        # Take first element of the queue
        v = que.popleft()
 
        # Take the possible value of K
        if (K < len(gr[v])):
            K = len(gr[v])
 
        # Current color
        cur = 1
 
        for u in gr[v]:
 
            # If vertex is already visited
            if (used[u]):
                continue
 
            # If the color is similar
            # to it's parent
            if (cur == cs[v]):
                cur += 1
 
            # Assign the color
            cs[u] = cur
            color[(u, v)] = color[(v, u)] = cur
            cur += 1
 
            # Mark it visited
            used[u] = 1
 
            # Push into the queue
            que.append(u)
 
    # Print minimum required colors
    print(K)
 
    # Print edge colors
    for p in edges:
        i = (p[0], p[1])
        print(color[i], end = " ")
 
# Driver code
n = 8
 
# Add edges
add_edge(0, 1)
add_edge(1, 2)
add_edge(1, 3)
add_edge(1, 4)
add_edge(3, 6)
add_edge(4, 5)
add_edge(5, 7)
 
# Function call
color_tree(n)
 
# This code is contributed by mohit kumar 29

C#




using System;
using System.Collections.Generic;
 
class Program
{
 
  // Add an edge between the vertexes
  static void AddEdge(List<List<int>> gr, int x, int y,
                      List<Tuple<int, int>> edges)
  {
    gr[x].Add(y);
    gr[y].Add(x);
    edges.Add(Tuple.Create(x, y));
  }
 
  // Function to color the tree with minimum number
  // of colors such that the colors of
  // the edges incident to a vertex are different
  static int ColorTree(int n, List<List<int>> gr,
                       List<Tuple<int, int>> edges)
  {
 
    // To store the minimum colors
    int K = 0;
 
    // To store color of the edges
    Dictionary<Tuple<int, int>, int> color = new Dictionary<Tuple<int, int>, int>();
 
    // Color of edge between its parent
    List<int> cs = new List<int>(new int[n]);
 
    // To check if the vertex is visited or not
    List<int> used = new List<int>(new int[n]);
 
    Queue<int> que = new Queue<int>();
    used[0] = 1;
    que.Enqueue(0);
 
    while (que.Count > 0) {
      // Take first element of the queue
      int v = que.Dequeue();
 
      // Take the possible value of K
      if (K < gr[v].Count)
        K = gr[v].Count;
 
      // Current color
      int cur = 1;
 
      foreach (int u in gr[v]) {
        // If vertex is already visited
        if (used[u] != 0)
          continue;
 
        // If the color is similar to it's parent
        if (cur == cs[v])
          cur++;
 
        // Assign the color
        cs[u] = color[Tuple.Create(u, v)]
          = color[Tuple.Create(v, u)] = cur++;
 
        // Mark it visited
        used[u] = 1;
 
        // Push into the queue
        que.Enqueue(u);
      }
    }
 
    // Print the minimum required colors
    Console.WriteLine(K);
 
    // Print the edge colors
    foreach (Tuple<int, int> p in edges)
      Console.Write(color[p] + " ");
 
    return K;
  }
 
  // Driver code
  static void Main() {
    int n = 8;
 
    List<List<int>> gr = new List<List<int>>();
    for (int i = 0; i < n; i++)
      gr.Add(new List<int>());
 
    List<Tuple<int, int>> edges = new List<Tuple<int, int>>();
 
    // Add edges
    AddEdge(gr, 0, 1, edges);
    AddEdge(gr, 1, 2, edges);
    AddEdge(gr, 1, 3, edges);
    AddEdge(gr, 1, 4, edges);
    AddEdge(gr, 3, 6, edges);
    AddEdge(gr, 4, 5, edges);
    AddEdge(gr, 5, 7, edges);
 
    // Function call
    ColorTree(n, gr, edges);
  }
}

Javascript




// Add an edge between the vertexes
function add_edge(gr, x, y, edges) {
  gr[x].push(y);
  gr[y].push(x);
  edges.push([x, y]);
}
 
// Function to color the tree with minimum
// number of colors such that the colors of
// the edges incident to a vertex are different
function color_tree(n, gr, edges) {
  // To store the minimum colors
  let K = 0;
 
  // To store color of the edges
  const color = new Map();
 
  // Color of edge between its parent
  const cs = new Array(n).fill(0);
 
  // To check if the vertex is
  // visited or not
  const used = new Array(n).fill(false);
 
  const que = [];
  used[0] = true;
  que.push(0);
 
  while (que.length > 0) {
    // Take first element of the queue
    const v = que.shift();
 
    // Take the possible value of K
    if (K < gr[v].length) {
      K = gr[v].length;
    }
 
    // Current color
    let cur = 1;
 
    for (const u of gr[v]) {
      // If vertex is already visited
      if (used[u]) {
        continue;
      }
 
      // If the color is similar to its parent
      if (cur === cs[v]) {
        cur++;
      }
 
      // Assign the color
      const edge = [u, v].sort();
      color.set(edge.toString(), cur);
      cs[u] = cur++;
 
      // Mark it visited
      used[u] = true;
 
      // Push into the queue
      que.push(u);
    }
  }
 
  // Print the minimum required colors
  console.log(K);
 
  // Print the edge colors
  for (const p of edges) {
    console.log(color.get(p.sort().toString()));
  }
}
 
// Driver code
function main() {
  const n = 8;
 
  const gr = new Array(n).fill(null).map(() => []);
  const edges = [];
 
  // Add edges
  add_edge(gr, 0, 1, edges);
  add_edge(gr, 1, 2, edges);
  add_edge(gr, 1, 3, edges);
  add_edge(gr, 1, 4, edges);
  add_edge(gr, 3, 6, edges);
  add_edge(gr, 4, 5, edges);
  add_edge(gr, 5, 7, edges);
 
  // Function call
  color_tree(n, gr, edges);
}
 
main();

Output:

4
1 2 3 4 1 1 2

The time complexity : O(n + e), where n is the number of vertices in the graph and e is the number of edges. This is because the code uses BFS to traverse the graph and visits each vertex and its edges once, which takes O(n + e) time.

The space complexity :  O(n + e), as it uses a queue and an array to store vertices, a map to store colors of edges, and an array to store colors of edges between the parent and the vertex.


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