# competitive-programming-library

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# strongly connected components decomposition, Kosaraju’s algorithm / 強連結成分分解 (graph/strongly_connected_components.hpp)

## Code

#pragma once
#include <functional>
#include <utility>
#include <vector>
#include "graph/transpose_graph.hpp"
#include "utils/macros.hpp"

/**
* @brief strongly connected components decomposition, Kosaraju's algorithm / 強連結成分分解
* @return the pair (the number k of components, the function from vertices of g to components)
* @param g is an adjacent list of a digraph
* @param g_rev is the transpose graph of g
* @note $O(V + E)$
*/
std::pair<int, std::vector<int> > decompose_to_strongly_connected_components(const std::vector<std::vector<int> > & g, const std::vector<std::vector<int> > & g_rev) {
int n = g.size();
std::vector<int> acc(n); {
std::vector<bool> used(n);
std::function<void (int)> dfs = [&](int i) {
used[i] = true;
for (int j : g[i]) if (not used[j]) dfs(j);
acc.push_back(i);
};
REP (i,n) if (not used[i]) dfs(i);
reverse(ALL(acc));
}
int size = 0;
std::vector<int> component_of(n); {
std::vector<bool> used(n);
std::function<void (int)> rdfs = [&](int i) {
used[i] = true;
component_of[i] = size;
for (int j : g_rev[i]) if (not used[j]) rdfs(j);
};
for (int i : acc) if (not used[i]) {
rdfs(i);
++ size;
}
}
return { size, move(component_of) };
}

std::pair<int, std::vector<int> > decompose_to_strongly_connected_components(const std::vector<std::vector<int> > & g) {
return decompose_to_strongly_connected_components(g, make_transpose_graph(g));
}



#line 2 "graph/strongly_connected_components.hpp"
#include <functional>
#include <utility>
#include <vector>
#line 2 "utils/macros.hpp"
#define REP(i, n) for (int i = 0; (i) < (int)(n); ++ (i))
#define REP3(i, m, n) for (int i = (m); (i) < (int)(n); ++ (i))
#define REP_R(i, n) for (int i = (int)(n) - 1; (i) >= 0; -- (i))
#define REP3R(i, m, n) for (int i = (int)(n) - 1; (i) >= (int)(m); -- (i))
#define ALL(x) std::begin(x), std::end(x)
#line 4 "graph/transpose_graph.hpp"

/**
* @param g is an adjacent list of a digraph
* @note $O(V + E)$
* @see https://en.wikipedia.org/wiki/Transpose_graph
*/
std::vector<std::vector<int> > make_transpose_graph(std::vector<std::vector<int> > const & g) {
int n = g.size();
std::vector<std::vector<int> > h(n);
REP (i, n) {
for (int j : g[i]) {
h[j].push_back(i);
}
}
return h;
}
#line 7 "graph/strongly_connected_components.hpp"

/**
* @brief strongly connected components decomposition, Kosaraju's algorithm / 強連結成分分解
* @return the pair (the number k of components, the function from vertices of g to components)
* @param g is an adjacent list of a digraph
* @param g_rev is the transpose graph of g
* @note $O(V + E)$
*/
std::pair<int, std::vector<int> > decompose_to_strongly_connected_components(const std::vector<std::vector<int> > & g, const std::vector<std::vector<int> > & g_rev) {
int n = g.size();
std::vector<int> acc(n); {
std::vector<bool> used(n);
std::function<void (int)> dfs = [&](int i) {
used[i] = true;
for (int j : g[i]) if (not used[j]) dfs(j);
acc.push_back(i);
};
REP (i,n) if (not used[i]) dfs(i);
reverse(ALL(acc));
}
int size = 0;
std::vector<int> component_of(n); {
std::vector<bool> used(n);
std::function<void (int)> rdfs = [&](int i) {
used[i] = true;
component_of[i] = size;
for (int j : g_rev[i]) if (not used[j]) rdfs(j);
};
for (int i : acc) if (not used[i]) {
rdfs(i);
++ size;
}
}
return { size, move(component_of) };
}

std::pair<int, std::vector<int> > decompose_to_strongly_connected_components(const std::vector<std::vector<int> > & g) {
return decompose_to_strongly_connected_components(g, make_transpose_graph(g));
}