LCA函数中

for (register inti=lg[depth[x]]-1; i>=0; --i)

这段代码改为

for (register inti=lg[depth[x]]-1; i; --i)

本地IDE运行会出错

请问为什么qwq

#include <bits/stdc++.h>
using namespace std;

int lg[10086];

int edge[10086], head[10086], nex[10086];
int tot;

void add (int x, int y)
{
	edge[++tot] = y; nex[tot] = head[x]; head[x] = tot;
	edge[++tot] = x; nex[tot] = head[y]; head[y] = tot;
}

int depth[10086], f[10086][10086];

void solve (int now, int father)
{
	f[now][0] = father;
	++depth[now] += depth[father];
	for (register int i=1; i<=lg[depth[now]]; ++i)
		f[now][i] = f[f[now][i-1]][i-1];
	for (register int i=head[now]; i; i=nex[i])
		if(edge[i]!=father)
			solve(edge[i], now);
}

int LCA (int x, int y)
{
	if (depth[x]<depth[y])
		swap(x, y);
	while (depth[x]>depth[y])
		x = f[x][lg[depth[x]-depth[y]]-1];
	if (x==y)
		return x;
	for (register int i=lg[depth[x]]-1; i>=0; --i)
		if (f[x][i]!=f[y][i])
			x = f[x][i],y=f[y][i];
	return f[x][0];
}

signed main (void)
{
	cin.tie(0);
	ios::sync_with_stdio(false);
	
	int n, m, s;
	cin >> n >> m >> s;
	
	for (register int i=1; i<n; ++i)
	{
		int x, y;
		cin >> x >> y;
		add(x,y);
	}
	
	lg[0] = -1;
	for (register int i=1; i<=n; ++i)
	{
		lg[i] = lg[i>>1] + 1;
	}
	solve(s,0);
	
	for (register int i=1; i<=m; ++i)
	{
		int x, y;
		cin >> x >> y;
		cout << LCA(x,y) << endl;
	}
	
	return 0;
}

LCA函数中

for (register inti=lg[depth[x]]-1; i>=0; --i)

这段代码改为

for (register inti=lg[depth[x]]-1; i; --i)

本地IDE运行会出错

请问为什么qwq