luogu P1131 时态同步

Posted 2021-01-18 yuyanjiab

传送门

题面贼长 题贼简单

修改边权使得所有叶子结点到根的路径长度相同

dfs的时候回溯修改成最大值即可

Code:

 1 // luogu-judger-enable-o2
 2 #include<cstdio>
 3 #include<algorithm>
 4 #include<cstring>
 5 #include<cstring>
 6 #define inf 2147483647
 7 #define rep(i,a,n) for(int i = a;i <= n;i++)
 8 #define per(i,n,a) for(int i = n;i >= a;i--)
 9 using namespace std;
10 typedef long long ll;
11 int read() {
12     int as = 0,fu = 1;
13     char c = getchar();
14     while(c<‘0‘||c>‘9‘) {
15         if(c == ‘-‘) fu = -1;
16         c = getchar();
17     }
18     while(c<=‘9‘&&c>=‘0‘) {
19         as = as * 10 + c - ‘0‘;
20         c = getchar();
21     }
22     return as * fu;
23 }
24 const int N = 1000005;
25 //head
26 int n,s;
27 int head[N],nxt[N],mo[N],cnt;
28 ll cst[N],stk[N],top,sum;
29 inline void add(int x,int y,ll z) {
30   mo[++cnt] = y;
31   cst[cnt] = z;
32   nxt[cnt] = head[x];
33   head[x] = cnt;
34   return;
35 }
36 ll dep[N];
37 
38 int dfs(int x,int f) {
39   for(int i = head[x];i;i = nxt[i]) {
40     int sn = mo[i];
41     if(sn == f)continue;
42     dfs(sn,x);
43     dep[x] = max(dep[sn] + cst[i],dep[x]);
44   }
45   for(int i = head[x];i;i = nxt[i]) {
46     int sn = mo[i];
47     if(sn == f) continue;
48     sum += dep[x] - dep[sn] - cst[i];
49   }
50   return dep[x];
51 }
52 
53 
54 int main() {
55   n = read();
56   s = read();
57   rep(i,1,n-1) {
58     int x = read();
59     int y = read();
60     ll z = read();
61     add(x,y,z);
62     add(y,x,z);
63   }
64   dfs(s,s);
65   printf("%lld
",sum);
66   return 0;
67 }