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树的重心分解简要模板
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struct centroid_decomposition { | |
vector<int> size; | |
vector<bool> used; | |
vector<int> fa; // 重心树 | |
int n; | |
const vector<vector<int>> &g; // vertices are 1-indexed | |
int centroid, cmx; // cmx: size of max block after removing centroid | |
void dfs(int u, int f, int n) { // 求重心 | |
size[u] = 1; | |
int mx = 0; | |
for (auto &v: g[u]) { | |
if (v != f && !used[v]) { | |
dfs(v, u, n); | |
size[u] += size[v]; | |
Max(mx, size[v]); | |
} | |
} | |
Max(mx, n - size[u]); | |
if (mx < cmx) { | |
centroid = u; | |
cmx = mx; | |
} | |
} | |
void divide(int u, int n, int c) { // c: 上次求出的重心 | |
cmx = INT_MAX; | |
dfs(u, 0, n); | |
fa[centroid] = c; // 构造重心树 | |
used[centroid] = true; | |
// 慎用全局变量 | |
c = centroid; | |
FOR(v, g[c]) { // 写成 FOR(v, g[centroid]) 也行,但是最好不要这样做! | |
if (!used[v]) { | |
int m = size[v] < size[c] ? size[v] : n - size[c]; // 这一步极容易错! | |
divide(v, m, c); | |
} | |
} | |
} | |
centroid_decomposition(int n, const vector<vector<int>> &g) : n(n), g(g) { | |
used.assign(n + 1, false); | |
size.resize(n + 1); | |
fa.resize(n + 1); | |
divide(1, n, 0); // | |
} | |
}; |
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