Dynamic Connectivity | Set 1 (Incremental)

Last Updated : 19 Jun, 2025

Dynamic connectivity is a data structure that dynamically maintains the information about the connected components of graph. In simple words suppose there is a graph G(V, E) in which number of vertices V is constant but number of edges E is variable. There are three ways in which we can change the number of edges

Incremental Connectivity : Edges are only added to the graph.
Decremental Connectivity : Edges are only deleted from the graph.
Fully Dynamic Connectivity : Edges can both be deleted and added to the graph.

In this article only Incremental connectivity is discussed. There are mainly two operations that need to be handled.

[1, u, v]: An edge between u and v is added to the graph.
[2, u, v]: Check if u and v are connected.

Example:

Input: V = 5, queries[][] = [[1, 0, 1], [2, 0, 1], [2, 1, 2], [1, 0, 2], [2, 1, 2], [1, 3, 4], [2, 2, 4], [1, 1, 3], [2, 2, 4]]
Output: true false true false true
Explanation: Initial state: Components = [[0], [1], [2], [3], [4]]
Query [1, 0, 1]: Union(0,1) --> Components = [[0,1], [2], [3], [4]]
Query [2, 0, 1]: Check connectivity --> Connected: true
Query [2, 1, 2]: Check connectivity --> Connected: false
Query [1, 0, 2]: Union(0,2) --> Components = [[0,1,2], [3], [4]]
Query [2, 1, 2]: Check connectivity --> Connected: true
Query [1, 3, 4]: Union(3,4) --> Components = [[0,1,2], [3,4]]
Query [2, 2, 4]: Check connectivity --> Connected: false
Query [1, 1, 3]: Union(1,3) --> Components = [[0,1,2,3,4]]
Query [2, 2, 4]: Check connectivity --> Connected: true

Approach:

To solve the problems of incremental connectivity disjoint data structure is used. Here each connected component represents a set and if the two nodes belong to the same set it means that they are connected.

Implementation is given below here we are using union by rank and path compression

C++

#include <bits/stdc++.h>
using namespace std;

// Find the representative of the set that x belongs to
int find(int i, vector<int> &parent) {
	int root = parent[i];

	if (parent[root] != root) {
		return parent[i] = find(root, parent);
	}

	return root;
}

// Union of sets containing x and y
void unionSets(int x, int y, vector<int> &rank, vector<int> &parent) {
	int xRoot = find(x, parent);
	int yRoot = find(y, parent);

	// If they are in the same set, no need to union
	if (xRoot == yRoot) return;

	// Union by rank
	if (rank[xRoot] < rank[yRoot]) {
		parent[xRoot] = yRoot;
	} else if (rank[yRoot] < rank[xRoot]) {
		parent[yRoot] = xRoot;
	} else {
		parent[yRoot] = xRoot;
		rank[xRoot]++;
	}
}

vector<bool> incrementalQueries(int V, vector<vector<int>> &queries) {
	vector<int> rank(V, 0);
	vector<int> parent(V);
	for (int i=0; i<V; i++) parent[i] = i;
	
	vector<bool> res;

    for (int i=0; i<queries.size(); i++) {
        int q = queries[i][0], u = queries[i][1],
        v = queries[i][2];
        
        // For union query 
        if (q == 1) {
            unionSets(u, v, rank, parent);
        }
        
        // Check if connected
        else {
            
            // If parents are same, then 
            // connected, otherwise not 
            if (find(u, parent) == find(v, parent)) {
                res.push_back(true);
            }
            else {
                res.push_back(false);
            }
        }
    }
    
    return res;
}

int main() {
	int V = 5;
	vector<vector<int>> queries =
	{	{1, 0, 1}, {2, 0, 1}, {2, 1, 2}, {1, 0, 2}, {2, 1, 2},
		{1, 3, 4}, {2, 2, 4}, {1, 1, 3}, {2, 2, 4}
	};
	vector<bool> res = incrementalQueries(V, queries);
	for (bool val: res) {
	    if (val) cout << "true" << " ";
	    else cout << "false" << " ";
	} 
	cout << endl;

	return 0;
}

Java

import java.util.ArrayList;

class GfG {
    
    // Find the representative of the set that x belongs to
    static int find(int i, int[] parent) {
        int root = parent[i];

        if (parent[root] != root) {
            return parent[i] = find(root, parent);
        }

        return root;
    }

    // Union of sets containing x and y
    static void unionSets(int x, int y, int[] rank, int[] parent) {
        int xRoot = find(x, parent);
        int yRoot = find(y, parent);

        // If they are in the same set, no need to union
        if (xRoot == yRoot) return;

        // Union by rank
        if (rank[xRoot] < rank[yRoot]) {
            parent[xRoot] = yRoot;
        } else if (rank[yRoot] < rank[xRoot]) {
            parent[yRoot] = xRoot;
        } else {
            parent[yRoot] = xRoot;
            rank[xRoot]++;
        }
    }

    static ArrayList<Boolean> incrementalQueries(int V, int[][] queries) {
        int[] rank = new int[V];
        int[] parent = new int[V];
        for (int i = 0; i < V; i++) parent[i] = i;
        
        ArrayList<Boolean> res = new ArrayList<>();

        for (int i = 0; i < queries.length; i++) {
            int q = queries[i][0], u = queries[i][1],
            v = queries[i][2];
            
            // For union query 
            if (q == 1) {
                unionSets(u, v, rank, parent);
            }
            
            // Check if connected
            else {
                
                // If parents are same, then 
                // connected, otherwise not 
                if (find(u, parent) == find(v, parent)) {
                    res.add(true);
                }
                else {
                    res.add(false);
                }
            }
        }
        
        return res;
    }

    public static void main(String[] args) {
        int V = 5;
        int[][] queries =
        {   {1, 0, 1}, {2, 0, 1}, {2, 1, 2}, {1, 0, 2}, {2, 1, 2},
            {1, 3, 4}, {2, 2, 4}, {1, 1, 3}, {2, 2, 4}
        };
        ArrayList<Boolean> res = incrementalQueries(V, queries);
        for (boolean val : res) {
            if (val) System.out.print("true ");
            else System.out.print("false ");
        } 
        System.out.println();
    }
}

Python

# Find the representative of the set that x belongs to
def find(i, parent):
    root = parent[i]

    if parent[root] != root:
        parent[i] = find(root, parent)
        return parent[i]

    return root

# Union of sets containing x and y
def unionSets(x, y, rank, parent):
    xRoot = find(x, parent)
    yRoot = find(y, parent)

    # If they are in the same set, no need to union
    if xRoot == yRoot:
        return

    # Union by rank
    if rank[xRoot] < rank[yRoot]:
        parent[xRoot] = yRoot
    elif rank[yRoot] < rank[xRoot]:
        parent[yRoot] = xRoot
    else:
        parent[yRoot] = xRoot
        rank[xRoot] += 1

def incrementalQueries(V, queries):
    rank = [0] * V
    parent = [i for i in range(V)]
    
    res = []

    for query in queries:
        q, u, v = query
        
        # For union query 
        if q == 1:
            unionSets(u, v, rank, parent)
        
        # Check if connected
        else:
            
            # If parents are same, then 
            # connected, otherwise not 
            if find(u, parent) == find(v, parent):
                res.append(True)
            else:
                res.append(False)
    
    return res

if __name__ == "__main__":
    V = 5
    queries = [
        [1, 0, 1], [2, 0, 1], [2, 1, 2], [1, 0, 2], [2, 1, 2],
        [1, 3, 4], [2, 2, 4], [1, 1, 3], [2, 2, 4]
    ]
    res = incrementalQueries(V, queries)
    for val in res:
        if val:
            print("true", end=" ")
        else:
            print("false", end=" ")
    print()

using System;
using System.Collections.Generic;

class GfG {
    
    // Find the representative of the set that x belongs to
    static int Find(int i, int[] parent) {
        int root = parent[i];

        if (parent[root] != root) {
            return parent[i] = Find(root, parent);
        }

        return root;
    }

    // Union of sets containing x and y
    static void UnionSets(int x, int y, int[] rank, int[] parent) {
        int xRoot = Find(x, parent);
        int yRoot = Find(y, parent);

        // If they are in the same set, no need to union
        if (xRoot == yRoot) return;

        // Union by rank
        if (rank[xRoot] < rank[yRoot]) {
            parent[xRoot] = yRoot;
        } else if (rank[yRoot] < rank[xRoot]) {
            parent[yRoot] = xRoot;
        } else {
            parent[yRoot] = xRoot;
            rank[xRoot]++;
        }
    }

    static List<bool> IncrementalQueries(int V, int[,] queries) {
        int[] rank = new int[V];
        int[] parent = new int[V];
        for (int i = 0; i < V; i++) parent[i] = i;
        
        List<bool> res = new List<bool>();

        for (int i = 0; i < queries.GetLength(0); i++) {
            int q = queries[i, 0], u = queries[i, 1],
            v = queries[i, 2];
            
            // For union query 
            if (q == 1) {
                UnionSets(u, v, rank, parent);
            }
            
            // Check if connected
            else {
                
                // If parents are same, then 
                // connected, otherwise not 
                if (Find(u, parent) == Find(v, parent)) {
                    res.Add(true);
                }
                else {
                    res.Add(false);
                }
            }
        }
        
        return res;
    }

    static void Main() {
        int V = 5;
        int[,] queries =
        {   {1, 0, 1}, {2, 0, 1}, {2, 1, 2}, {1, 0, 2}, {2, 1, 2},
            {1, 3, 4}, {2, 2, 4}, {1, 1, 3}, {2, 2, 4}
        };
        List<bool> res = IncrementalQueries(V, queries);
        foreach (bool val in res) {
            if (val) Console.Write("true ");
            else Console.Write("false ");
        } 
        Console.WriteLine();
    }
}

JavaScript

// Find the representative of the set that x belongs to
function find(i, parent) {
    let root = parent[i];

    if (parent[root] !== root) {
        return parent[i] = find(root, parent);
    }

    return root;
}

// Union of sets containing x and y
function unionSets(x, y, rank, parent) {
    let xRoot = find(x, parent);
    let yRoot = find(y, parent);

    // If they are in the same set, no need to union
    if (xRoot === yRoot) return;

    // Union by rank
    if (rank[xRoot] < rank[yRoot]) {
        parent[xRoot] = yRoot;
    } else if (rank[yRoot] < rank[xRoot]) {
        parent[yRoot] = xRoot;
    } else {
        parent[yRoot] = xRoot;
        rank[xRoot]++;
    }
}

function incrementalQueries(V, queries) {
    let rank = new Array(V).fill(0);
    let parent = new Array(V);
    for (let i = 0; i < V; i++) parent[i] = i;
    
    let res = [];

    for (let i = 0; i < queries.length; i++) {
        let q = queries[i][0], u = queries[i][1],
        v = queries[i][2];
        
        // For union query 
        if (q === 1) {
            unionSets(u, v, rank, parent);
        }
        
        // Check if connected
        else {
            
            // If parents are same, then 
            // connected, otherwise not 
            if (find(u, parent) === find(v, parent)) {
                res.push(true);
            }
            else {
                res.push(false);
            }
        }
    }
    
    return res;
}

let V = 5;
let queries = [
    [1, 0, 1], [2, 0, 1], [2, 1, 2], [1, 0, 2], [2, 1, 2],
    [1, 3, 4], [2, 2, 4], [1, 1, 3], [2, 2, 4]
];
let res = incrementalQueries(V, queries);
for (let val of res) {
    if (val) process.stdout.write("true ");
    else process.stdout.write("false ");
} 
console.log();

Output

true false true false true

Time Complexity: O(α(n)), Inverse Ackermann, nearly constant time, because of path compression and union by rank optimization.
Space Complexity: O(n), For parent and rank arrays as arrays store disjoint set info for n elements.

Dynamic Connectivity | Set 1 (Incremental)

Ayush Jha

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Dynamic Connectivity | Set 1 (Incremental)

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