poj 1330 Nearest Common Ancestors（LCA）

代码

#include <cstdio>
#include <cstring>
#include <vector>
#include <queue>
#include <algorithm>

using namespace std;
const int maxn = 1e4 + 5;
const int inf = 0x3f3f3f3f;
const int BIT = 20;

int dep[maxn], f[maxn][BIT+5];
vector<int> G[maxn];

void ST (int n) {
for (int k = 1; k <= BIT; k++) {
for (int i = 1; i <= n; i++)
f[i][k] = f[f[i][k-1]][k-1];
}

int s;
for (int i = 1; i <= n; i++) if (f[i][0] == 0) {
s = i; break;
}
memset(dep, inf, sizeof(dep));
dep[s] = 0;

queue<int> que;
que.push(s);
while (!que.empty()) {
int u = que.front();
que.pop();

for (int i = 0; i < G[u].size(); i++) {
int v = G[u][i];
if (dep[v] > dep[u] + 1) {
dep[v] = dep[u] + 1;
que.push(v);
}
}
}
}

int LCA(int u, int v) {
if (dep[u] > dep[v]) swap(u, v);

int mv = dep[v] - dep[u];
for (int i = 0; i <= BIT && mv; mv >>= 1, i++)
if (mv&1) v = f[v][i];

if (u == v) return u;

for (int i = BIT; i >= 0; i--) {
if (f[u][i] == f[v][i]) continue;
u = f[u][i]; v = f[v][i];
}
return f[u][0];
}

int main () {
int cas;
scanf("%d", &cas);
while (cas--) {
memset(f, 0, sizeof(f));

int n, u, v;
scanf("%d", &n);
for (int i = 1; i <= n; i++) G[i].clear();
for (int i = 1; i < n; i++) {
scanf("%d%d", &u, &v);
f[v][0] = u;
G[u].push_back(v);
}
ST(n);
scanf("%d%d", &u, &v);
printf("%d\n", LCA(u, v));
}
return 0;
}