CF379F New Year Tree

题意

给定图：

每次在叶子结点加入两个点，并实时输出树的直径长度。

思路

每次增加两个点，直径至多变化一个点，长度最多加 1，所以对加入的点处理 lca，并且更新长度和点即可。

代码

#include <bits/stdc++.h>using namespace std;const int N = 1000010;int fa[30][N], dep[N];void process(int u) {for (int i = 1; i < 30; i++) {fa[i][u] = fa[i - 1][fa[i - 1][u]];}
}int lca(int u, int v) {if (dep[u] < dep[v]) swap(u, v);for (int j = 29; j >= 0; j--) {if (dep[fa[j][u]] >= dep[v]) u = fa[j][u];}if (u == v) return u;for (int j = 29; j >= 0; j--) {if (fa[j][u] != fa[j][v]) {u = fa[j][u];v = fa[j][v];}}return fa[0][u];
}int dis(int u, int v) {return dep[u] + dep[v] - 2 * dep[lca(u, v)];
}int main() {ios::sync_with_stdio(false);cin.tie(nullptr);int q;cin >> q;fa[0][2] = fa[0][3] = fa[0][4] = 1;dep[2] = dep[3] = dep[4] = 1;int a1 = 2, a2 = 3, res = 2, ver = 5;for (int opt = 1; opt <= q; opt++) {int x;cin >> x;fa[0][ver] = fa[0][ver + 1] = x;dep[ver] = dep[ver + 1] = dep[x] + 1;process(ver);process(ver + 1);int len1 = dis(a1, ver);int len2 = dis(a2, ver);int len3 = dis(a1, ver + 1);int len4 = dis(a2, ver + 1);if (len1 > res) {res = len1;a2 = ver;}else if (len2 > res) {res = len2;a1 = ver;}else if (len3 > res) {res = len3;a2 = ver + 1;}else if (len4 > res) {res = len4;a1 = ver + 1;}cout << res << '\n';ver += 2;}return 0;
}