SPOJ/PT07X - 1435. Vertex Cover/pt07x.cpp

   1 // Andrés Mejía
   2 using namespace std;
   3 #include <algorithm>
   4 #include <iostream>
   5 #include <iterator>
   6 #include <numeric>
   7 #include <sstream>
   8 #include <fstream>
   9 #include <cassert>
  10 #include <climits>
  11 #include <cstdlib>
  12 #include <cstring>
  13 #include <string>
  14 #include <cstdio>
  15 #include <vector>
  16 #include <cmath>
  17 #include <queue>
  18 #include <deque>
  19 #include <stack>
  20 #include <list>
  21 #include <map>
  22 #include <set>
  23
  24 #define foreach(x, v) for (typeof (v).begin() x=(v).begin(); x !=(v).end(); ++x)
  25 #define For(i, a, b) for (int i=(a); i<(b); ++i)
  26 #define D(x) cout << #x " is " << x << endl
  27
  28 const double EPS = 1e-9;
  29 int cmp(double x, double y = 0, double tol = EPS) {
  30     return (x <= y + tol) ? (x + tol < y) ? -1 : 0 : 1;
  31 }
  32
  33 const int MAXN = 100001;
  34 vector<int> g[MAXN];
  35 int memo[MAXN][2];
  36
  37 int f(int node, bool edge, int parent = -1) {
  38     assert(g[node].size() > 0);
  39     if (g[node].size() == 1 and g[node][0] == parent) { // Base case
  40         return edge ? 0 : 1;
  41     }
  42
  43     if (memo[node][edge] > -1) return memo[node][edge];
  44
  45     // take this node
  46     int ans = 1;
  47     for (int k = 0; k < g[node].size(); ++k) {
  48         int v = g[node][k];
  49         if (v == parent) continue;
  50         ans += f(v, true, node);
  51     }
  52
  53     if (edge) {
  54         // do not take this node
  55         int option = 0;
  56         for (int k = 0; k < g[node].size(); ++k) {
  57             int v = g[node][k];
  58             if (v == parent) continue;
  59             option += f(v, false, node);
  60         }
  61         ans = min(ans, option);
  62     }
  63
  64     return memo[node][edge] = ans;
  65 }
  66
  67 int main(){
  68     int n; scanf("%d", &n);
  69     for (int i = 0; i < n; ++i) {
  70          g[i].clear();
  71          memo[i][0] = memo[i][1] = -1;
  72      }
  73     for (int i = 0; i < n - 1; ++i) {
  74         int u, v; scanf("%d %d", &u, &v);
  75         u--; v--;
  76         g[u].push_back(v);
  77         g[v].push_back(u);
  78     }
  79     if (n == 1) {
  80         printf("0\n");
  81     } else {
  82         printf("%d\n", min(f(0, false), f(0, true)));
  83     }
  84     return 0;
  85 }