Презентация на тему Shortest paths and spanning trees in graphs

2015

between two vertices such that the sum of the

weights of edges in path is minimized.
Known algorithms:
Dijkstra
Floyd–Warshall
Bellman–Ford
and so on...

visited and unvisited.
For visited vertices we know minimal distance

from start. For unvisited vertices we know some distance which can be not minimal.
Initially, all vertices are unvisited and distance to each vertex is INF. Only distance to start node is equal 0.
On each step choose unvisited vertex with minimal distance. Now it’s visited vertex. And try to relax distance of neighbors.

Complexity: trivial implementation O(|V|^2+|E|)
implementation with set O(|E|log|V|+|V|log|V|)

= 0;
for (int i = 0; i < n;

++i)
{
int u = -1;
for (int j = 0; j < n; ++j)
if (!mark[j] && (u == -1 || d[j] < d[u]))
u = j;
mark[u] = true;
for (v - сосед u)
d[v] = min(d[v], d[u] + weight(uv));
}
}

//(dist[u], u)
vector dist(n, INF);
dist[s] = 0;
q.insert(mp(0, s));
while(!q.empty())
{
int cur =

q.begin()->second;
q.erase(q.begin());
for(auto e : g[cur])
if(dist[e.first] > dist[cur]+e.second)
{
q.erase(mp(dist[e.first], e.first));
dist[e.first] = dist[cur] + e.second;
q.insert(mp(dist[e.first], e.first));
}
}
}

q; //(dist[u], u)
vector dist(n, INF);
dist[s] = 0;
q.push(mp(0, s));
while(!q.empty())
{
int cur

= q.top().second;
int cur_d = -q.top().first; q.pop();
if(cur_d > dist[cur]) continue;
for(auto e : g[cur])
if(dist[e.first] > dist[cur]+e.second)
{
dist[e.first] = dist[cur] + e.second;
q.push(mp(-dist[e.first], e.first));
}
}
}

v): dist[u][v]=weight(u, v)
On iteration k we let use vertex

k as intermediate vertex and for each pair of vertices we try to relax distance.
dist[u][v] = min(dist[u][v], dist[u][k]+dist[k][v])

Complexity: O(|V|^3)

= 0; i < n; ++i)
dist[i][i] = 0;
for (int

i = 0; i < n; ++i)
for (auto e : g[i])
dist[i][e.first] = e.second;
for (int k = 0; k < n; ++k)
for (int i = 0; i < n; ++i)
for (int j = 0; j < n; ++j)
dist[i][j] = min(dist[i][j], dist[i][k] + dist[k][j]);
}

distance with all edges.
If we can relax distance on

|V| iteration then negative cycle exists in graph
Why |V|-1 iterations? Because the longest way without cycles from one node to another one contains no more |V|-1 edges.
Complexity O(|V||E|)

i = 0; i < n - 1; ++i)
for

(auto e : edges)
dist[e.v] = min(dist[e.v], dist[e.u] + e.weight);

for (auto e : edges)
if (dist[e.v] > dist[e.u] + e.weight)
cout << "Negative cycle!" << endl;
}

undirected graph G is a subgraph that includes all

of the vertices of G that is a tree.
A minimal spanning tree is a spanning tree and sum of weights is minimized.

chosen arbitrarily from the graph.
Grow the tree by one

edge: of the edges that connect the tree to vertices not yet in the tree, find the minimum-weight edge, transfer it to the tree and try to relax distance for neighbors.
Repeat step 2 (until all vertices are in the tree).

Complexity: trivial implementation O(|V|^2+|E|)
implementation with set O(|E|log|V|+|E|)

INF);
dist[0] = 0;
q.insert(mp(0, 0));
while (!q.empty())
{
int cur = q.begin()->second;
q.erase(q.begin());
for (auto

e : g[cur])
if (dist[e.first] > e.second)
{
q.erase(mp(dist[e.first], e.first));
dist[e.first] = e.second;
q.insert(mp(dist[e.first], e.first));
}
}
}

trees), where each vertex in the graph is a

separate tree
Create a set S containing all the edges in the graph
While S is nonempty and F is not yet spanning:
remove an edge with minimum weight from S
if the removed edge connects two different trees then add it to the forest F, combining two trees into a single tree

Complexity: trivial O(|V|^2+|E|log|E|)
with DSU O(|E|log|E|)

i < n; ++i)
color[i] = i;
sort(edges.begin(), edges.end());
for(auto e :

edges)
if(color[e.u]!=color[e.v])
{
add_to_spanning_tree(e);
int c1 = color[e.u];
int c2 = color[e.v];
for (int i = 0; i < n; ++i)
if (color[i] == c1)
color[i] = c2;
}
}

edges)
if(dsu.findSet(e.u)!=dsu.findSet(e.v))
{
add_to_spanning_tree(e);
dsu.unionSets(e.u, e.v);
}
}

set, which contains U, complexity O(1)
Union(U, V) – join

sets, which contain U and V, complexity O(1)

After creating DSU:
After some operations:

0; i < n; ++i)
p[i] = i;
}

int find(int u)

{
return u == p[u] ? u : find(p[u]);
}

void merge(int u, int v) {
int pu = find(u);
int pv = find(v);
p[pv] = pu;
}
};

root of set for each vertex in path
int findSet(int

u)
{
return u == p[u] ? u : p[u] = findSet(p[u]);
}

= findSet(u);
int pv = findSet(v);
if (pu == pv) return;
if

(sizes[pu] < sizes[pv])
swap(pu, pv);
p[pv] = pu;
sizes[pu] += sizes[pv];
}

DSU(int size)
{
p.resize(size);
sizes.resize(size, 1);
for (int i = 0; i < size; ++i)
p[i] = i;
}