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  1. /* cerberus97 - Hanit Banga */
  2.  
  3. #include <iostream>
  4. #include <iomanip>
  5. #include <cassert>
  6. #include <cmath>
  7. #include <cstdio>
  8. #include <cstring>
  9. #include <cstdlib>
  10. #include <map>
  11. #include <set>
  12. #include <queue>
  13. #include <stack>
  14. #include <vector>
  15. #include <algorithm>
  16.  
  17. using namespace std;
  18.  
  19. #define pb push_back
  20. #define fast_cin() ios_base::sync_with_stdio(false); cin.tie(NULL)
  21.  
  22. typedef long long ll;
  23. typedef long double ld;
  24. typedef pair <int, int> pii;
  25. typedef pair <ll, ll> pll;
  26.  
  27.  
  28. // 2-SAT solver based on Kosaraju's algorithm.
  29. // Variables are 0-based. Positive variables are stored in vertices 2n, corresponding negative variables in 2n+1
  30. // TODO: This is quite slow (3x-4x slower than Gabow's algorithm)
  31. struct TwoSat {
  32. 	int n;
  33. 	vector<vector<int> > adj, radj, scc;
  34. 	vector<int> sid, vis, val;
  35. 	stack<int> stk;
  36. 	int scnt;
  37.  
  38. 	// n: number of variables, including negations
  39. 	TwoSat(int n): n(n), adj(n), radj(n), sid(n), vis(n), val(n, -1) {}
  40.  
  41. 	// adds an implication
  42. 	void impl(int x, int y) { adj[x].push_back(y); radj[y].push_back(x); }
  43. 	// adds a disjunction
  44. 	void vee(int x, int y) { impl(x^1, y); impl(y^1, x); }
  45. 	// forces variables to be equal
  46. 	void eq(int x, int y) { impl(x, y); impl(y, x); impl(x^1, y^1); impl(y^1, x^1); }
  47. 	// forces variable to be true
  48. 	void tru(int x) { impl(x^1, x); }
  49.  
  50. 	void dfs1(int x) {
  51. 		if (vis[x]++) return;
  52. 		for (int i = 0; i < adj[x].size(); i++) {
  53. 			dfs1(adj[x][i]);
  54. 		}
  55. 		stk.push(x);
  56. 	}
  57.  
  58. 	void dfs2(int x) {
  59. 		if (!vis[x]) return; vis[x] = 0;
  60. 		sid[x] = scnt; scc.back().push_back(x);
  61. 		for (int i = 0; i < radj[x].size(); i++) {
  62. 			dfs2(radj[x][i]);
  63. 		}
  64. 	}
  65.  
  66. 	// returns true if satisfiable, false otherwise
  67. 	// on completion, val[x] is the assigned value of variable x
  68. 	// note, val[x] = 0 implies val[x^1] = 1
  69. 	bool two_sat() {
  70. 		scnt = 0;
  71. 		for (int i = 0; i < n; i++) {
  72. 			dfs1(i);
  73. 		}
  74. 		while (!stk.empty()) {
  75. 			int v = stk.top(); stk.pop();
  76. 			if (vis[v]) {
  77. 				scc.push_back(vector<int>());
  78. 				dfs2(v);
  79. 				scnt++;
  80. 			}
  81. 		}
  82. 		for (int i = 0; i < n; i += 2) {
  83. 			if (sid[i] == sid[i+1]) return false;
  84. 		}
  85. 		vector<int> must(scnt);
  86. 		for (int i = 0; i < scnt; i++) {
  87. 			for (int j = 0; j < scc[i].size(); j++) {
  88. 				val[scc[i][j]] = must[i];
  89. 				must[sid[scc[i][j]^1]] = !must[i];
  90. 			}
  91. 		}
  92. 		return true;
  93. 	}
  94. };
  95.  
  96. int main() {
  97. 	fast_cin();
  98. 	TwoSat TS(6);
  99. 	TS.impl(0, 4);
  100. 	TS.impl(2, 5);
  101. 	cout << TS.two_sat() << endl;
  102. 	for (int i = 0; i < 6; ++i) {
  103. 		cout << i << ' ' << TS.val[i] << endl;
  104. 	}
  105. }
Success #stdin #stdout 0s 4580KB
comments (?)
stdin 
Standard input is empty
stdout 
1
0 1
1 0
2 1
3 0
4 1
5 0
https://ideone.com/H5J8Vr
language:
C++14 (gcc 8.3)
created:
4 years ago
visibility:
public

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