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| #include <iostream>
#include <cstdio>
#include <cctype>
#include <cstring>
#define maxn 300010
#define gc getchar
using namespace std;
int read() {
int x = 0; char c = gc();
while (!isdigit(c)) c = gc();
while (isdigit(c)) x = x * 10 + c - '0', c = gc();
return x;
}
int n, m;
struct Edge {
int to, next, w;
} e[maxn * 2]; int c1, head[maxn];
inline void add_edge(int u, int v, int w) {
e[c1].to = v; e[c1].w = w;
e[c1].next = head[u]; head[u] = c1++;
}
struct Query {
int u, v, lca;
} Q[maxn];
int f[maxn][21], dis[maxn], dep[maxn], _w[maxn];
void dfs(int u, int fa) {
for (int i = head[u]; ~i; i = e[i].next) {
int v = e[i].to, w = e[i].w; if (v == fa) continue;
dis[v] = dis[u] + w; f[v][0] = u; dep[v] = dep[u] + 1;
_w[v] = w; dfs(v, u);
}
}
void init_lca() {
for (int j = 1; j <= 20; ++j)
for (int i = 1; i <= n; ++i) f[i][j] = f[f[i][j - 1]][j - 1];
}
inline int get_lca(int x, int y) {
if (dep[x] < dep[y]) swap(x, y);
for (int i = 20; ~i; --i)
if (dep[f[x][i]] >= dep[y]) x = f[x][i];
if (x == y) return x;
for (int i = 20; ~i; --i)
if (f[x][i] != f[y][i]) x = f[x][i], y = f[y][i];
return f[x][0];
}
int Dis[maxn], Max, tot;
void Dfs(int u, int fa) {
for (int i = head[u]; ~i; i = e[i].next) {
int v = e[i].to; if (v == fa) continue;
Dfs(v, u); Dis[u] += Dis[v];
}
if (Dis[u] == tot) Max = max(Max, _w[u]);
}
bool check(int x) {
memset(Dis, 0, sizeof Dis); int Mx = 0; tot = Max = 0;
for (int i = 1; i <= m; ++i) {
int u = Q[i].u, v = Q[i].v, lca = Q[i].lca;
if (dis[u] + dis[v] - 2 * dis[lca] <= x) continue;
Mx = max(Mx, dis[u] + dis[v] - 2 * dis[lca]);
++Dis[u]; ++Dis[v]; Dis[lca] -= 2; ++tot;
}
Dfs(1, 0);
return Mx - Max <= x;
}
int main() { memset(head, -1, sizeof head);
n = read(); m = read();
for (int i = 1; i < n; ++i) {
int x = read(), y = read(), z = read();
add_edge(x, y, z); add_edge(y, x, z);
}
for (int i = 1; i <= m; ++i) Q[i].u = read(), Q[i].v = read();
dep[1] = 1; dfs(1, 0); init_lca(); int Max = 0;
for (int i = 1; i <= m; ++i) {
int u = Q[i].u, v = Q[i].v, lca;
lca = Q[i].lca = get_lca(u, v);
Max = max(Max, dis[u] + dis[v] - 2 * dis[lca]);
}
int l = 0, r = Max, mid, ans;
while (l <= r) {
mid = l + r >> 1;
if (check(mid)) ans = mid, r = mid - 1;
else l = mid + 1;
} cout << ans << endl;
return 0;
}
|