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find-the-weak-connected-component-in-the-directed-graph.cpp
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find-the-weak-connected-component-in-the-directed-graph.cpp
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// Time: O(nlogn)
// Space: O(n)
/**
* Definition for Directed graph.
* struct DirectedGraphNode {
* int label;
* vector<DirectedGraphNode *> neighbors;
* DirectedGraphNode(int x) : label(x) {};
* };
*/
class Solution {
public:
/**
* @param nodes a array of directed graph node
* @return a connected set of a directed graph
*/
vector<vector<int>> connectedSet2(vector<DirectedGraphNode*>& nodes) {
// Sort nodes by its index. Time: O(nlogn)
sort(nodes.begin(), nodes.end(),
[](const DirectedGraphNode* a, const DirectedGraphNode* b) {
return a->label < b->label;
});
// The disjoint-set.
unordered_map<int, int> set;
// Initialize the disjoint-set.
for (const auto& node : nodes) {
set[node->label] = node->label;
}
// Union-Find each edge.
for (const auto& node : nodes) {
for (const auto& neighbor : node->neighbors) {
union_set(&set, node->label, neighbor->label);
}
}
// Group each component.
unordered_map<int, vector<int>> group;
for (const auto& node : nodes) {
group[find_set(node->label, &set)].emplace_back(node->label);
}
// Sort elements in each group.
vector<vector<int>> result;
for (auto& kvp : group) {
result.emplace_back(move(kvp.second));
}
return result;
}
int find_set(int x, unordered_map<int, int> *set) {
if ((*set)[x] != x) {
(*set)[x] = find_set((*set)[x], set); // path compression.
}
return (*set)[x];
}
void union_set(unordered_map<int, int> *set, int x, int y) {
int x_root = find_set(x, set), y_root = find_set(y, set);
(*set)[min(x_root, y_root)] = max(x_root, y_root);
}
};