Tree
Time Limit: 1000MS | Memory Limit: 30000K | |
Total Submissions: 3506 | Accepted: 1204 |
Description
Consider a tree with N vertices, numbered from 1 to N. Add, if it is possible, a minimum number of edges thus every vertex belongs to exactly one cycle.
Input
The input has the following structure:
N
x(1) y(1)
x(2) y(2)
...
x(N-1) y(n-1)
N (3 <= N <=100) is the number of vertices. x(i) and y(i) (x(i), y(i) are integers, 1 <= x(i), y(i) <= N) represent the two vertices connected by the i-th edge.
N
x(1) y(1)
x(2) y(2)
...
x(N-1) y(n-1)
N (3 <= N <=100) is the number of vertices. x(i) and y(i) (x(i), y(i) are integers, 1 <= x(i), y(i) <= N) represent the two vertices connected by the i-th edge.
Output
The output will contain the value -1 if the problem doesn't have a solution, otherwise an integer, representing the number of added edges.
Sample Input
7 1 2 1 3 3 5 3 4 5 6 5 7
Sample Output
2
Source
树形DP啊,可是知道也推不出公式来,。无奈的看了解题报告。发现前方的道路还非常漫长啊,这思路神了
http://www.cnblogs.com/vongang/archive/2012/08/12/2634763.html
#include <iostream> #include <cstring> #include <cstdio> #include <algorithm> #include <cstdlib> #include <cmath> #define N 110 #define INF 150 using namespace std; struct num { int y,next; }a[N*N]; int b[N],Top; int dp[N][3]; bool ch[N]; int main() { //freopen("data.txt","r",stdin); void addeage(int x,int y); void dfs(int x); int n; while(scanf("%d",&n)!=EOF) { Top = 0; memset(b,-1,sizeof(b)); for(int i=1;i<=n-1;i++) { int x,y; scanf("%d %d",&x,&y); addeage(x,y); addeage(y,x); } for(int i=1;i<=n;i++) { dp[i][0] = dp[i][1] = dp[i][2] = INF; } memset(ch,false,sizeof(ch)); dfs(1); //cout<<dp[3][1]<<endl; if(dp[1][0]==INF) { printf("%d\n",-1); }else { printf("%d\n",dp[1][0]); } } return 0; } void addeage(int x,int y) { a[Top].y = y; a[Top].next = b[x]; b[x] = Top++; } void dfs(int x) { ch[x] = true; bool uv = true; for(int i=b[x];i!=-1;i=a[i].next) { int y = a[i].y; if(!ch[y]) { uv = false; break; } } if(uv) { dp[x][1] =0; return ; } dp[x][1] = 0; int sum = 0; bool ch2[N]; memset(ch2,false,sizeof(ch2)); for(int i=b[x];i!=-1;i=a[i].next) { int y = a[i].y; if(!ch[y]) { dfs(y); ch2[y] = true; dp[x][1]+=dp[y][0]; sum+=dp[y][0]; } } if(dp[x][1]>=INF) { dp[x][1] = INF; } for(int i=b[x];i!=-1;i=a[i].next) { int y = a[i].y; if(!ch2[y]) { continue; } int w = sum-dp[y][0]+min(dp[y][1],dp[y][2]); if(w>=INF) { w = INF; } dp[x][2] = min(dp[x][2],w); w = sum-dp[y][0] + dp[y][2]+1; if(w>=INF) { w = INF; } dp[x][0] = min(dp[x][0],w); } for(int i=b[x];i!=-1;i=a[i].next) { int y1 = a[i].y; if(!ch2[y1]) { continue; } for(int j=b[x];j!=-1;j=a[j].next) { int y2 = a[j].y; if(y1==y2||!ch2[y2]) { continue; } int w = sum-dp[y1][0] - dp[y2][0] + min(dp[y1][1],dp[y1][2]) + min(dp[y2][1],dp[y2][2])+1; if(w>=INF) { w = INF; } dp[x][0] = min(dp[x][0],w); } } }