# Minimum nodes to be colored in a Graph such that every node has a colored neighbour

Given a graph **G **with **V **nodes and **E** edges, the task is to colour no more than **floor(V/2)** nodes such that every node has *at least one coloured node* at a distance of atmost 1 unit. The distance between any two connected nodes of the graph is always exactly 1 unit. Print the nodes that need to be colored.

**Examples:**

Input:N = 4,

G:

Output:1

Input:N = 6,

G:

Output:3

**Approach:** This problem can be solved using **BFS** traversal. Follow the steps below to solve the problem:

- Initialize arrays
**odd[]**and**even[]**to store nodes which are at a distance of odd and even number of nodes respectively from the source. - Starting from the source node, perform BFS traversal with
**distance**initialized 0, which denotes the distance from source node. Store all the nodes at a particular level in odd[] or even[] based on the value of**distance**. - If distance is odd, that is
**(distance & 1)**is 1, insert that node into odd[]. Otherwise, insert into even[]. - Now print the nodes from the array with minimum elements.
- Since minimum among count of nodes at odd distance or even distance does not exceed
**floor(V/2)**, the answer holds correct as every node at odd distance is connected to nodes at even distance and vice-versa. - Hence, if number of nodes at even distance from source is less, print the nodes from even[]. Otherwise, print all nodes from odd[].

Illustration:

For the graph G given below,

Source Node S = 1

- even[] = {1, 3, 5}
- odd[] = {2, 6, 4}
- minimum(even.size(), odd.size()) = min(3, 3) = 3
- Hence, coloring either all nodes at odd distance from the source or even distance from the source is correct as both their count is same.

Below is the implementation of the above approach:

## C++

`// C++ Program to implement the` `// above approach` `#include <bits/stdc++.h>` `using` `namespace` `std;` `// Stores the graph` `map<` `int` `, vector<` `int` `> > graph;` `// Stores the visited nodes` `map<` `int` `, ` `int` `> vis;` `// Stores the nodes` `// at odd distance` `vector<` `int` `> odd;` `// Stores the nodes at` `// even distance` `vector<` `int` `> even;` `// Function to separate and` `// store the odd and even` `// distant nodes from source` `void` `bfs()` `{` ` ` `// Source node` ` ` `int` `src = 1;` ` ` `// Stores the nodes and their` ` ` `// respective distances from` ` ` `// the source` ` ` `queue<pair<` `int` `, ` `int` `> > q;` ` ` `// Insert the source` ` ` `q.push({ src, 0 });` ` ` `// Mark the source visited` ` ` `vis[src] = 1;` ` ` `while` `(!q.empty()) {` ` ` `// Extract a node from the` ` ` `// front of the queue` ` ` `int` `node = q.front().first;` ` ` `int` `dist = q.front().second;` ` ` `q.pop();` ` ` `// If distance from source` ` ` `// is odd` ` ` `if` `(dist & 1) {` ` ` `odd.push_back(node);` ` ` `}` ` ` `// Otherwise` ` ` `else` `{` ` ` `even.push_back(node);` ` ` `}` ` ` `// Traverse its neighbors` ` ` `for` `(` `auto` `i : graph[node]) {` ` ` `// Insert its unvisited` ` ` `// neighbours into the queue` ` ` `if` `(!vis.count(i)) {` ` ` `q.push({ i, (dist + 1) });` ` ` `vis[i] = 1;` ` ` `}` ` ` `}` ` ` `}` `}` `// Driver Program` `int` `main()` `{` ` ` `graph[1].push_back(2);` ` ` `graph[2].push_back(1);` ` ` `graph[2].push_back(3);` ` ` `graph[3].push_back(2);` ` ` `graph[3].push_back(4);` ` ` `graph[4].push_back(3);` ` ` `graph[4].push_back(5);` ` ` `graph[5].push_back(4);` ` ` `graph[5].push_back(6);` ` ` `graph[6].push_back(5);` ` ` `graph[6].push_back(1);` ` ` `graph[1].push_back(6);` ` ` `bfs();` ` ` `if` `(odd.size() < even.size()) {` ` ` `for` `(` `int` `i : odd) {` ` ` `cout << i << ` `" "` `;` ` ` `}` ` ` `}` ` ` `else` `{` ` ` `for` `(` `int` `i : even) {` ` ` `cout << i << ` `" "` `;` ` ` `}` ` ` `}` ` ` `return` `0;` `}` |

## Java

`// Java program to implement the` `// above approach` `import` `java.util.ArrayList;` `import` `java.util.HashMap;` `import` `java.util.LinkedList;` `import` `java.util.Map;` `import` `java.util.Queue;` `class` `Pair` `{` ` ` `int` `first, second;` ` ` `public` `Pair(` `int` `first, ` `int` `second)` ` ` `{` ` ` `this` `.first = first;` ` ` `this` `.second = second;` ` ` `}` `}` `class` `GFG{` `// Stores the graph` `static` `Map<Integer, ArrayList<Integer>> graph = ` `new` `HashMap<>();` `// Stores the visited nodes` `static` `Map<Integer, Integer> vis = ` `new` `HashMap<>();` `// Stores the nodes` `// at odd distance` `static` `ArrayList<Integer> odd = ` `new` `ArrayList<>();` `// Stores the nodes at` `// even distance` `static` `ArrayList<Integer> even = ` `new` `ArrayList<>();` `// Function to separate and` `// store the odd and even` `// distant nodes from source` `static` `void` `bfs()` `{` ` ` ` ` `// Source node` ` ` `int` `src = ` `1` `;` ` ` `// Stores the nodes and their` ` ` `// respective distances from` ` ` `// the source` ` ` `Queue<Pair> q = ` `new` `LinkedList<>();` ` ` `// Insert the source` ` ` `q.add(` `new` `Pair(src, ` `0` `));` ` ` `// Mark the source visited` ` ` `vis.put(src, ` `1` `);` ` ` `while` `(!q.isEmpty())` ` ` `{` ` ` ` ` `// Extract a node from the` ` ` `// front of the queue` ` ` `int` `node = q.peek().first;` ` ` `int` `dist = q.peek().second;` ` ` `q.poll();` ` ` `// If distance from source` ` ` `// is odd` ` ` `if` `((dist & ` `1` `) != ` `0` `)` ` ` `{` ` ` `odd.add(node);` ` ` `}` ` ` `// Otherwise` ` ` `else` ` ` `{` ` ` `even.add(node);` ` ` `}` ` ` `// Traverse its neighbors` ` ` `for` `(Integer i : graph.get(node))` ` ` `{` ` ` ` ` `// Insert its unvisited` ` ` `// neighbours into the queue` ` ` `if` `(!vis.containsKey(i))` ` ` `{` ` ` `q.add(` `new` `Pair(i, (dist + ` `1` `)));` ` ` `vis.put(i, ` `1` `);` ` ` `}` ` ` `}` ` ` `}` `}` `// Driver code` `public` `static` `void` `main(String[] args)` `{` ` ` `graph.put(` `1` `, ` `new` `ArrayList<>());` ` ` `graph.put(` `2` `, ` `new` `ArrayList<>());` ` ` `graph.put(` `3` `, ` `new` `ArrayList<>());` ` ` `graph.put(` `4` `, ` `new` `ArrayList<>());` ` ` `graph.put(` `5` `, ` `new` `ArrayList<>());` ` ` `graph.put(` `6` `, ` `new` `ArrayList<>());` ` ` `graph.get(` `1` `).add(` `2` `);` ` ` `graph.get(` `2` `).add(` `1` `);` ` ` `graph.get(` `2` `).add(` `3` `);` ` ` `graph.get(` `3` `).add(` `2` `);` ` ` `graph.get(` `3` `).add(` `4` `);` ` ` `graph.get(` `4` `).add(` `3` `);` ` ` `graph.get(` `4` `).add(` `5` `);` ` ` `graph.get(` `5` `).add(` `4` `);` ` ` `graph.get(` `5` `).add(` `6` `);` ` ` `graph.get(` `6` `).add(` `5` `);` ` ` `graph.get(` `6` `).add(` `1` `);` ` ` `graph.get(` `1` `).add(` `6` `);` ` ` `bfs();` ` ` `if` `(odd.size() < even.size())` ` ` `{` ` ` `for` `(` `int` `i : odd)` ` ` `{` ` ` `System.out.print(i + ` `" "` `);` ` ` `}` ` ` `}` ` ` `else` ` ` `{` ` ` `for` `(` `int` `i : even)` ` ` `{` ` ` `System.out.print(i + ` `" "` `);` ` ` `}` ` ` `}` `}` `}` `// This code is contributed by sanjeev2552` |

## Python3

`# Python3 Program to implement the` `# above approach` ` ` `# Stores the graph` `graph ` `=` `dict` `()` ` ` `# Stores the visited nodes` `vis ` `=` `dict` `()` ` ` `# Stores the nodes` `# at odd distance` `odd ` `=` `[]` ` ` `# Stores the nodes at` `# even distance` `even ` `=` `[]` ` ` `# Function to separate and` `# store the odd and even` `# distant nodes from source` `def` `bfs():` ` ` `# Source node` ` ` `src ` `=` `1` `;` ` ` ` ` `# Stores the nodes and their` ` ` `# respective distances from` ` ` `# the source` ` ` `q ` `=` `[]` ` ` ` ` `# Insert the source` ` ` `q.append([ src, ` `0` `]);` ` ` ` ` `# Mark the source visited` ` ` `vis[src] ` `=` `1` `;` ` ` ` ` `while` `(` `len` `(q) !` `=` `0` `):` ` ` ` ` `# Extract a node from the` ` ` `# front of the queue` ` ` `node ` `=` `q[` `0` `][` `0` `]` ` ` `dist ` `=` `q[` `0` `][` `1` `]` ` ` `q.pop(` `0` `);` ` ` ` ` `# If distance from source` ` ` `# is odd` ` ` `if` `(dist & ` `1` `):` ` ` `odd.append(node);` ` ` ` ` `# Otherwise` ` ` `else` `:` ` ` `even.append(node);` ` ` ` ` `# Traverse its neighbors` ` ` `for` `i ` `in` `graph[node]:` ` ` ` ` `# Insert its unvisited` ` ` `# neighbours into the queue` ` ` `if` `(i ` `not` `in` `vis):` ` ` ` ` `q.append([ i, (dist ` `+` `1` `) ]);` ` ` `vis[i] ` `=` `1` `;` ` ` `# Driver code` `if` `__name__` `=` `=` `'__main__'` `:` ` ` `graph[` `1` `] ` `=` `[` `2` `, ` `6` `]` ` ` `graph[` `2` `] ` `=` `[` `1` `, ` `3` `]` ` ` `graph[` `3` `] ` `=` `[` `2` `, ` `4` `]` ` ` `graph[` `4` `] ` `=` `[` `3` `, ` `5` `]` ` ` `graph[` `5` `] ` `=` `[` `4` `, ` `6` `]` ` ` `graph[` `6` `] ` `=` `[` `5` `, ` `1` `]` ` ` ` ` `bfs();` ` ` ` ` `if` `(` `len` `(odd) < ` `len` `(even)):` ` ` `for` `i ` `in` `odd:` ` ` `print` `(i, end ` `=` `' '` `)` ` ` ` ` `else` `:` ` ` `for` `i ` `in` `even:` ` ` `print` `(i, end ` `=` `' '` `)` ` ` `# This code is contributed by rutvik_56` |

**Output:**

1 3 5

**Time Complexity:** O(V + E)

**Auxiliary Space**: O(V)

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