Three Paths on a Tree 樹的直徑
阿新 • • 發佈:2021-01-06
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題目描述
在一棵樹內找三個點x,y,z,要求三個點之間的距離最大,
分析
首先我們可以找到這棵樹內的直徑,然後去去找另一點到這棵樹上的直徑最短
我一開始的思路是三次bfs,求出直徑的端點,並且處理出每一個點到直徑兩個端點的距離,然後去給每一個符合d1[x] + d2[x] == d (d為樹的直徑)的點打上標記,然後亂搞,後來發現這樣做麻煩了
我們直接求每一個點到直徑兩端的距離,然後再加上直徑的長度,這樣其實就是每個距離多加了一遍,最後整體除以2就可以了
程式碼
#include <iostream>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <queue>
#include <cstring>
#define debug(x) cout<<#x<<":"<<x<<endl;
#define _CRT_SECURE_NO_WARNINGS
#pragma GCC optimize("Ofast","unroll-loops","omit-frame-pointer","inline")
#pragma GCC option("arch=native","tune=native","no-zero-upper")
#pragma GCC target("avx2")
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> PII;
const int INF = 0x3f3f3f3f;
const int N = 2e5 + 10,M = 4e5 + 10;
int h[ N],ne[M],e[M],idx;
int st[N];
int is[N];
int d1[N],d2[N];
int d[N];
int n;
void add(int x,int y){
ne[idx] = h[x],e[idx] = y,h[x] = idx++;
}
int main(){
int x = 0,y = 0,z = 0;
int ans = 0;
memset(h,-1,sizeof h);
scanf("%d",&n);
for(int i = 1;i < n;i++){
int x,y;
scanf("%d%d",&x,&y);
add(x,y),add(y,x);
}
queue<int> Q;
Q.push(1);
st[1] = true;
while(Q.size()){
int t = Q.front();
Q.pop();
for(int i = h[t];~i;i = ne[i]){
int j = e[i];
if(st[j]) continue;
d1[j] = d1[t] + 1;
st[j] = true;
Q.push(j);
}
}
for(int i = 1;i <= n;i++)
if(d1[i] > d1[x]) x = i;
Q.push(x);
memset(st,0,sizeof st);
st[x] = true;
while(Q.size()){
int t = Q.front();
Q.pop();
for(int i = h[t];~i;i = ne[i]){
int j = e[i];
if(st[j]) continue;
st[j] = true;
d2[j] = d2[t] + 1;
Q.push(j);
}
}
for(int i = 1;i <= n;i++)
if(d2[i] > d2[y]) y = i;
Q.push(y);
memset(st,0,sizeof st);
st[y] = true;
d1[y] = 0;
while(Q.size()){
int t = Q.front();
Q.pop();
for(int i = h[t];~i;i = ne[i]){
int j = e[i];
if(st[j]) continue;
st[j] = true;
d1[j] = d1[t] + 1;
Q.push(j);
}
}
int back = 0;
for(int i = 1;i <= n;i++){
if(i == x || i == y) continue;
back = (d1[i] + d2[i] + d1[x]) / 2;
if(back > ans){
ans = back;
z = i;
}
}
printf("%d\n",ans);
printf("%d %d %d\n",x,y,z);
return 0;
}
/**
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* ┃ ┗━━━┓ 神獸保佑,程式碼無bug
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*/