「Note」倍增LCA

没想到吧我已经连LCA都不能一次过了
不过我应该一直就是这个水平

不能折叠有点可惜，
倍增可以根据两个点的深度差直接找到起始跳跃层数
不要把 while 跟 for + if 弄混， while 停下来就走不了了。。虽然这种错误听起来很rz的 yysyqs

写代码确实要多思考啊。。

倍增 70pts 常数巨大代码巨丑版
#include<cstdio>
#include<cstdlib>
#include<cmath>
#include<cstring>
#include<iostream>
#include<algorithm>
using namespace std;
const int MAXM = 1e6 + 100;
const int MAXN = 5e5 + 50;
int n, m, s, tot = 0;
int nxt[MAXM], head[MAXM], to[MAXM], fa[MAXN][22], Dep[MAXN];
int TempPos, cnt;
void Tree(int Pos)
{
	TempPos = fa[Pos][0]; cnt = 0;
	while (fa[TempPos][cnt])
	{
		TempPos = fa[TempPos][cnt];
		fa[Pos][++cnt] = TempPos;
	}
	for (int i = head[Pos]; i; i = nxt[i])
	{
		if (fa[Pos][0]!=to[i])
		{
			Dep[to[i]] = Dep[Pos] + 1;
			fa[to[i]][0] = Pos;
			Tree(to[i]);
		}
	}
}
int LCA_Query(int a, int b)
{
	if (a==s||b==s) return s;
	if (a==b) return a;
	int TpA = a, TpB = b, tmp = 19;
	if (Dep[TpA] < Dep[TpB]) swap(TpA, TpB);
	if (Dep[TpA] > Dep[TpB])
	{
		while (tmp >= 0 && ((!fa[TpA][tmp]) || Dep[fa[TpA][tmp]] < Dep[TpB])) --tmp;
//		cout<<fa[TpA][tmp]<<"*(FD\n";
		while (tmp >= 0) 
		{
			if (Dep[fa[TpA][tmp]] >= Dep[TpB]) TpA = fa[TpA][tmp];
			--tmp;
//			cout<<tmp<<" "<<fa[TpA][tmp]<<" "<<Dep[fa[TpA][tmp]]<<" "<<Dep[TpB]<<"*(AFD\n";
		}
//	if(Dep[TpA]!=Dep[TpB])cout<<"&"<<TpA<<" "<<TpB<<" "<<Dep[TpA]<<" "<<Dep[TpB]<<" "<<fa[TpA][tmp]<<" "<<Dep[fa[TpA][tmp]]<<" "<<tmp<<endl;
	}
	if (TpA==TpB) return TpA;
	tmp = 19;
	while (!fa[TpA][tmp]) --tmp;
	while (tmp >= 0)
	{
		if (fa[TpA][tmp]!=fa[TpB][tmp]) TpA = fa[TpA][tmp], TpB = fa[TpB][tmp];
		--tmp;
	}
//	if (fa[TpA][0]==0) cout<<"&"<<TpA<<" "<<TpB<<" "<<Dep[TpA]<<" "<<Dep[TpB]<<endl;
	return fa[TpA][0];
}
int main()
{
	freopen("P3379_2 (2).in", "r", stdin);
	freopen("P3379_2 (3).out", "w", stdout);
	memset(fa, 0, sizeof(fa));
	memset(Dep, 0, sizeof(Dep));
	memset(head, 0, sizeof(head));
	memset(nxt, 0, sizeof(nxt));
	memset(to, 0, sizeof(to));
	cin >> n >> m >> s;
	for (int x, y, i = 1; i < n; ++i)
	{
		cin >> x >> y;
		++tot; to[tot] = y, nxt[tot] = head[x], head[x] = tot;
		++tot; to[tot] = x, nxt[tot] = head[y], head[y] = tot;
	}
	Dep[s] = 1;
	Tree(s);
//	for (int i = 1; i <= n; ++i) cout << i << " " << Dep[i] << endl;
	for (int a, b, i = 1; i <= m; ++i)
	{
		cin >> a >> b;
		cout << LCA_Query(a, b) << endl;
	}
	return 0;
}

预处理log2的话 复杂度可能少一个log。。

倍增 100pts
#include<cstdio>
#include<iostream>
#include<algorithm>
#include<cstdlib>
#include<cstring>
#include<cmath>
using namespace std;

#define add_edge(s, t) {nxt[++tot]=head[s], to[tot]=t, head[s]=tot;}
const int MAXN = 5e5+5, MAXM = 1e6+10; 
int tot = 0, nxt[MAXM], to[MAXM], head[MAXN], Dep[MAXN];
int fa[MAXN][21], I0G2[MAXN];
//int TmpPos, cnt;
int tmp;

void Build_Tree(int Pos)
{
	for (int i = 0; i < I0G2[Dep[Pos]]; ++i)
	{
		fa[Pos][i+1] = fa[fa[Pos][i]][i];
	}
	for (int i = head[Pos]; i; i = nxt[i])
	{
		if (fa[Pos][0] == to[i]) continue;
		fa[to[i]][0] = Pos; Dep[to[i]] = Dep[Pos] + 1;
		Build_Tree(to[i]);
	}
}

int LCA_Query(int a, int b)
{
	if (Dep[a] < Dep[b]) swap(a, b);
	tmp = I0G2[Dep[a] - Dep[b]];
	for (register int i = tmp; i >= 0; --i)
	{
		if (Dep[fa[a][i]]>=Dep[b]) a=fa[a][i];
	}
	if (a==b) return a; //*
	else for (register int i = I0G2[Dep[a]]; i >= 0; --i)
	{
		if (fa[a][i] != fa[b][i])
		{
			a = fa[a][i], b=fa[b][i];
		}
	}
	return fa[a][0];
}

int main()
{	
//	freopen("P3379_2 (2).in", "r", stdin);
//	freopen("P3379_2 (4).out", "w", stdout);
	
	int N, M, S;
	scanf("%d%d%d", &N, &M, &S);
	
	for (register int i = 1; i <= N; ++i)
	{
		I0G2[i] = I0G2[i>>1] + 1;
	}
	
	for (register int x, y, i = 1; i < N; ++i)
	{
		scanf("%d%d", &x, &y);
		add_edge(x, y); add_edge(y, x);
	}
	Dep[S]=1;
	Build_Tree(S);
	
	for (register int a, b, i = 1; i <= M; ++i)
	{
		scanf("%d%d", &a, &b);
		printf("%d\n", LCA_Query(a, b));
	}
	
	return 0;
}