# Kruskal's algorithm for Minimum Spanning Tree

July 04, 2018

Given an undirected weighted graph, a minimum spanning tree (MST) is a subset of the edges of the graph which form a tree and have the minimum total edge weight. For a MST to exist, the graph must be connected (that is, every pair of nodes must be reachable from each other).

Example of Minimum Spanning Tree. Total edge weight = 5 + 8 + 8 + 4 + 11 + 6 = 42

Kruskal’s algorithm build’s the minimum spanning tree one edge at a time. To see what that means, watch the following video.

# Algorithm steps

2. Iterate over the edges sorted by weight.
3. If adding edge (u, v) to the current tree does not create a cycle, then add it, else discard it.
4. Done!

# How to check for a cycle?

Given a graph with $n$ nodes and $m$ edges, if we use an adjacency list representation for the current tree, and BFS / DFS to see if adding edge (u, v) forms a cycle, then the algorithm complexity is O($mn$).

However, checking if adding (u, v) forms a cycle can be done in O($\log n$) using a disjoint-set data structure, which gives a run-time complexity of O($m \log n$).

# Bonus: Disjoint-set data-structures

The following is an efficient implementation of disjoint-set data-structures in Python along-with a detailed explanation in the comments.

from collections import defaultdict
import heapq
# each "set" is a tree, and the "set representative" is the tree root
# hence, two nodes are in the same set if root(u) == root(v)
# initially, everything is in its own set. hence parent(node) = node
n = 5
parent = list(range(n))
size = [1]*n
# to find the root, start from the node and keep going to parent[node]. stop when parent[node] = node.
# in addition, we set "parent[node] = root(node)" so that next time we look for root(node), we'll get there in 1 step!
def root(node):
if not parent[node] == node:
parent[node] = root(parent[node])
return parent[node]
# to merge the sets, we can simply do parent[root(u)] = root(v)
# to ensure that tree heights are O(log n), we make the root of the smaller tree a child of the root of the larger tree
# (since a node's root can't change > log n times)
def merge(uu, vv):
root_uu, root_vv = root(uu), root(vv)
assert root_uu != root_vv, (root_uu, root_vv)
if size[root_uu] < size[root_vv]:
parent[root_uu] = root_vv
size[root_uu] += size[root_vv]
else:
parent[root_vv] = root_uu
size[root_vv] += size[root_uu]
# initialisation
graph = defaultdict(list)
dist = [float('inf')]*n
cost = 0
mst = []
edges = [(0, 1), (1, 2), (2, 3), (3, 4), (4, 0), (1, 4), (1, 3)]
weight = [1, 4, 2, 6, 7, 5, 3];
sortedlist = []
for i in range (0, len(weight)):
sortedlist.append((weight[i], edges[i]))
# sorts in ascending order of weights of edges
sortedlist.sort()
for i in range(0, len(sortedlist)):
# If the two nodes are in disjoint sets then the
# roots would be different, so we merge them
if(root(sortedlist[i][1][0]) != root(sortedlist[i][1][1])):
merge(sortedlist[i][1][0], sortedlist[i][1][1])
cost += sortedlist[i][0]
mst.append((sortedlist[i][1][0], sortedlist[i][1][1]))
print("Minimum cost of spanning tree is: " + str(cost))
print("Following are the edges in MST: ")
for i in range(0,len(mst)):
print(str(mst[i][0]) + " " + str(mst[i][1])) 

The following is the cpp code for Kruskal’s implementation to find minimum spanning tree:

#include <bits/stdc++.h>
using namespace std;
int n,m;
vector<int>rt, sz;
vector<pair<int, int> >mst;
void init()
{
/*
Initialises the root of every
node as itself and size
disjoint set as 1
*/
for (int i = 0; i < n; i++)
{
rt[i] = i;
sz[i] = 1;
}
}
int root(int i)
{
while(i != rt[i])
{
rt[i] = rt[rt[i]];
i = rt[i];
}
return i;
}
void join(int a, int b)
{
int rta = root(a);
int rtb = root(b);
int sza = sz[rta];
int szb = sz[rtb];
/*
To merge the sets, we can simply do parent[root(u)] = root(v).
To ensure that tree heights are O(log n), we make the root of
the smaller tree a child of the root of the larger tree.
(since a node's root can't change more than log n times)
*/
if(sza >= szb)
{
rt[rtb] = rt[rta];
sz[rta] += sz[rtb];
sz[rtb] = 0;
}
else
{
rt[rta] = rt[rtb];
sz[rtb] += sz[rta];
sz[rta] = 0;
}
}
int main()
{
n = 5;
pair<int, int>edges[] = { {0, 1}, {1, 2}, {2, 3}, {3, 4}, {4, 0}, {1, 4}, {1, 3} };
int weight[] = {1, 4, 2, 6, 7, 5, 3};
m = sizeof(weight) / sizeof(weight[0]);
rt.resize(n);
sz.resize(n);
init();
vector<pair<int, int> >v;
for (int i = 0; i < m; i++)
{
v.push_back({weight[i], i});
}
// sorts in ascending order of weights of edges
sort(v.begin(), v.end());
int cost = 0;
for (int i = 0; i < m; i++)
{
int idx = v[i].second;
/* If the two nodes are in disjoint sets
then the roots would be different, in which
case we join the two nodes
*/
if (root(edges[idx].first) != root(edges[idx].second))
{
join(edges[idx].first, edges[idx].second);
cost += v[i].first;
mst.push_back({edges[idx].first, edges[idx].second});
}
}
cout << "Minimum cost of spanning tree is: " << cost << endl;
cout << "Following are the edges in MST: " << endl;
for(auto i: mst)
cout << i.first << " " << i.second << endl;
return 0;
}

# Correctness of Kruskal’s algorithm

## The cut property

The main idea behind both Prim’s algorithm and Kruskal’s algorithm is the cut property.

Consider any subset S of all vertices V of the graph. Consider all edges going from a vertex in S to a vertex outside S. The cut property states that, if $e$ is the edge with minimum weight among all these edges, then $e$ must be part of the minimum spanning tree.

Proof of cut property: The proof is quite simple. Assume the contrary, that is, the MST of the graph does not include $e$. Since the minimum spanning tree spans all vertices, it has at-least one edge $e'$ going from S to outside S. If this edge is replaced by $e$, we get a spanning tree with lesser weight. This is a contradiction, since the original tree was a minimum spanning tree.

## Correctness of Kruskal’s algorithm

In Kruskal’s algorithm, in each step, we consider edges sorted by weight. We include an edge only if connecting it’s end-points don’t form a cycle with already chosen edges.

Let the current edge be (u, v). Consider S = set of all edges u is connected to via edges already chosen in the MST. The current edge (u, v) is the minimum-weight edge from set S to all vertices outside S. Hence, (u, v) must be part of the MST.