学习树链剖分的第一题，第二个dfs忘记递归了（太蠢），re了两发，改过来以后就1A了。

学习树链剖分可以参考这篇博客：http://blog.sina.com.cn/s/blog_7a1746820100wp67.html 个人感觉写得非常好！！！

树链剖分听起来很吊，其实就是把数边扔到线段树上，然后搞出一点事情来。主要就是一个重儿子，重链，从而使复杂度降到了(logn)。这个非常关键。关于初始化建树：先剖分，求出size,son,w,top,fa,dep数组，就能把树链扔到线段树树上实现了，这个过程也就是两个dfs实现。然后就是简单的区间修改的问题了。（再次感觉那篇博客写得太好了）

树的边权的问题可以转化为树链剖分的问题！！！

#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <cmath>
#include <map>
#define LL long long
#define FOR(i,x,y)  for(int i = x;i < y;i ++)
#define IFOR(i,x,y) for(int i = x;i > y;i --)
#define lrt rt<<1
#define rrt rt<<1|1
#define lson rt<<1,l,mid
#define rson rt<<1|1,mid+1,r
#define MAXN 11000using namespace std;int n,val[MAXN];int head[MAXN<<1],edge_cnt;
map <int,int> mat;struct Edge{int u,v,idd,value;int nt;
}edge[MAXN<<1];void add_edge(int u,int v,int idd){edge[edge_cnt].u = u;edge[edge_cnt].v = v;edge[edge_cnt].idd = idd;edge[edge_cnt].nt = head[u];head[u] = edge_cnt++;
}int sz[MAXN],son[MAXN],fa[MAXN],dep[MAXN],top[MAXN],id[MAXN],w[MAXN],val0[MAXN],totw;void dfs1(int u,int father,int depth){sz[u] = 1;  dep[u] = depth;  fa[u] = father;int idd = -1,maxx = -1;for(int i = head[u];i != -1;i = edge[i].nt){int v = edge[i].v;if(v == father)  continue;id[v] = edge[i].idd;dfs1(v,u,depth+1);sz[u] += sz[v];if(sz[v] > maxx)    {idd = v;maxx = sz[v];}}son[u] = idd;
}void dfs2(int u,int father){if(son[u] == -1)    return;top[son[u]] = top[u];w[son[u]] = ++totw;val[totw] = val0[id[son[u]]];mat[id[son[u]]] = totw;dfs2(son[u],u);for(int i = head[u];i != -1;i = edge[i].nt){int v = edge[i].v;if(v == father) continue;if(v == son[u]) continue;top[v] = v;w[v] = ++totw;val[totw] = val0[id[v]];mat[id[v]] = totw;dfs2(v,u);}
}struct Tree{int l,r;int maxx;
}tree[MAXN<<2];void PushUp(int rt){tree[rt].maxx = max(tree[lrt].maxx,tree[rrt].maxx);
}void Build(int rt,int l,int r){tree[rt].l = l; tree[rt].r = r;if(l == r){tree[rt].maxx = val[l];return;}int mid = (l+r)>>1;Build(lson);Build(rson);PushUp(rt);
}void Modify(int rt,int x,int value){if(tree[rt].l == tree[rt].r){tree[rt].maxx = value;return;}int mid = (tree[rt].l + tree[rt].r)>>1;if(x <= mid)    Modify(lrt,x,value);else Modify(rrt,x,value);PushUp(rt);
}int Query(int rt,int l,int r){if(tree[rt].l == l && tree[rt].r == r){return tree[rt].maxx;}int mid = (tree[rt].l + tree[rt].r)>>1;if(r <= mid)    return Query(lrt,l,r);else if(l > mid)    return Query(rrt,l,r);else return max(Query(lson),Query(rson));
}int solve(int l,int r){int ans = -1;while(top[l] != top[r]){if(dep[top[l]] >= dep[top[r]]){ans = max(ans,Query(1,w[top[l]],w[l]));l = fa[top[l]];}else{ans = max(ans,Query(1,w[top[r]],w[r]));r = fa[top[r]];}}if(l == r)  return ans;if(dep[l] >= dep[r]){return max(ans,Query(1,w[r]+1,w[l]));}else return max(ans,Query(1,w[l]+1,w[r]));
}int main(){//freopen("test.in","r",stdin);int T;scanf("%d",&T);while(T--){scanf("%d",&n);int u,v,value;memset(head,-1,sizeof(head));edge_cnt = 0;FOR(i,1,n){scanf("%d%d%d",&u,&v,&value);add_edge(u,v,i);add_edge(v,u,i);val0[i] = value;}dfs1(1,-1,1);totw = 0;top[1] = 1;dfs2(1,-1);Build(1,1,totw);char op[10];while(~scanf("%s",op) && strcmp(op,"DONE")){if(strcmp(op,"QUERY") == 0){int l,r;scanf("%d%d",&l,&r);printf("%d\n",solve(l,r));}else{int x,value;scanf("%d%d",&x,&value);Modify(1,mat[x],value);}}}return 0;
}