题意:求树的重心。树的重心为:去掉树上某节点形成的多棵树中节点的个数存在最大值,求这些最大值的最小值。如果这样的最小值有多个,那么输出序号最小的节点。直观理解,删除重心后,剩余的多棵树尽可能平衡。(3107:如果树有多个重心,求出所有的重心,并按照从小到大的顺序求出;2378:求把点删掉剩下的子树中最大的节点数不大于总点数一半的点)
思路:将树表示成有向树。设一共有n个节点,节点p引出t棵子树,每棵子树有节点son[1],son[2]...son[t]个。则此节点的值为max(son[1],son[2]...son[t],n-son[p]-1)。
用dfs来求son数组,dfs的过程中更新结果。
#include <stdio.h> #include <string.h> #define max(a,b) a>b?a:b #define N 20005 struct node{ int y,next; }e[N*2]; int son[N],first[N],top,n,T,res1,res2; void init(){ memset(son,0,sizeof(son)); memset(first,-1,sizeof(first)); top = 0; res2 = 0x3fffffff; } void add(int x,int y){ e[top].y = y; e[top].next = first[x]; first[x] = top++; } void dfs(int x,int father){ int i,num=0; for(i = first[x];i!=-1;i=e[i].next) if(e[i].y != father){ dfs(e[i].y,x); son[x] += son[e[i].y] + 1; num = max(num,son[e[i].y]+1); } num = max(num,n-son[x]-1); if(num < res2||(num<=res2&&x<res1)){ res1 = x; res2 = num; } } int main(){ freopen("a.txt","r",stdin); scanf("%d",&T); while(T--){ int i,x,y; init(); scanf("%d",&n); for(i = 1;i<n;i++){ scanf("%d %d",&x,&y); add(x,y); add(y,x); } dfs(1,0); printf("%d %d\n",res1,res2); } return 0; }
3107:
#include <stdio.h> #include <string.h> #include <stdlib.h> #define max(a,b) ((a)>(b)?(a):(b)) #define N 50005 struct edge{ int y,next; }e[N<<1]; int n,top,m,len=0; int first[N],son[N],res[N]; void add(int x,int y){ e[top].y = y; e[top].next = first[x]; first[x] = top++; } int cmp(const void* a,const void* b){ return (*(int*)a)-(*(int*)b); } void dfs(int x,int fa){ int i,y,num; son[x] = 1; num = 0; for(i = first[x];i!=-1;i=e[i].next){ y = e[i].y; if(y!=fa && !son[y]){ dfs(y, x); son[x] += son[y]; num = max(num,son[y]); } } num = max(num, n-son[x]); if(num <= m){ if(num < m){ len = 0; m = num; } res[len++] = x; } } int main(){ int i,a,b; memset(first, -1, sizeof(first)); memset(son, 0, sizeof(son)); top = 0; scanf("%d",&n); for(i = 1;i<n;i++){ scanf("%d %d",&a,&b); add(a,b); add(b,a); } m = n; dfs(1,0); qsort(res,len,sizeof(int),cmp); for(i = 0;i<len-1;i++) printf("%d ",res[i]); printf("%d\n",res[len-1]); return 0; }
2378:
#include <stdio.h> #include <string.h> #include <stdlib.h> #define max(a,b) ((a)>(b)?(a):(b)) #define N 10005 struct edge{ int y,next; }e[N<<1]; int n,top,len=0; int first[N],son[N],res[N]; void add(int x,int y){ e[top].y = y; e[top].next = first[x]; first[x] = top++; } int cmp(const void* a,const void* b){ return (*(int*)a)-(*(int*)b); } void dfs(int x,int fa){ int i,y,num; son[x] = 1; num = 0; for(i = first[x];i!=-1;i=e[i].next){ y = e[i].y; if(y!=fa && !son[y]){ dfs(y, x); son[x] += son[y]; num = max(num,son[y]); } } num = max(num, n-son[x]); if(num <= n/2) res[len++] = x; } int main(){ int i,a,b; top = 0; scanf("%d",&n); memset(first, -1, n*sizeof(int)); memset(son, 0, n*sizeof(int)); for(i = 1;i<n;i++){ scanf("%d %d",&a,&b); add(a,b); add(b,a); } dfs(1,0); qsort(res, len, sizeof(int), cmp); for(i = 0;i<len;i++) printf("%d\n",res[i]); return 0; }