利用树的前序和中序递归判定最小公共祖先~

直接根据两个序列递归处理~

#include<bits/stdc++.h>
using namespace std;
const int maxn=;
int N,M;
int pre[maxn],in[maxn];
unordered_map<int,int> pos;
void lca (int inL,int inR,int preRoot,int a,int b) {
if (inL>inR) return;
int inRoot=pos[pre[preRoot]];
int aIn=pos[a];
int bIn=pos[b];
if ((aIn>inRoot&&bIn<inRoot)||(aIn<inRoot&&bIn>inRoot))
printf ("LCA of %d and %d is %d.\n",a,b,in[inRoot]);
else if (aIn>inRoot&&bIn>inRoot)
lca (inRoot+,inR,preRoot+inRoot-inL+,a,b);
else if (aIn<inRoot&&bIn<inRoot)
lca (inL,inRoot-,preRoot+,a,b);
else if (aIn==inRoot)
printf ("%d is an ancestor of %d.\n",a,b);
else if (bIn==inRoot)
printf ("%d is an ancestor of %d.\n",b,a);
}
int main () {
scanf ("%d %d",&M,&N);
for (int i=;i<=N;i++) {
scanf ("%d",&in[i]);
pos[in[i]]=i;
}
for (int i=;i<=N;i++)
scanf ("%d",&pre[i]);
int a,b;
for (int i=;i<M;i++) {
scanf ("%d %d",&a,&b);
if (pos[a]==&&pos[b]==)
printf ("ERROR: %d and %d are not found.\n",a,b);
else if (pos[a]==)
printf ("ERROR: %d is not found.\n",a);
else if (pos[b]==)
printf ("ERROR: %d is not found.\n",b);
else lca (,N,,a,b);
}
return ;
}

也可以根据树建图,跑lca算法

#include<bits/stdc++.h>
using namespace std;
const int maxn=;
struct node {
int data;
node * left;
node * right;
};
int M,N;
int pre[maxn],in[maxn];
unordered_map<int,int> pos;
int father[maxn*];
int depth[maxn*];
node * create (int preL,int preR,int inL,int inR) {
if (preL>preR) return NULL;
node * root=new node;
root->data=pre[preL];
int k;
for (k=inL;k<=inR;k++)
if (in[k]==pre[preL]) break;
int numLeft=k-inL;
root->left=create(preL+,preL+numLeft,inL,k-);
if (root->left!=NULL) father[root->left->data]=root->data;
root->right=create(preL+numLeft+,preR,k+,inR);
if (root->right!=NULL) father[root->right->data]=root->data;
return root;
}
void bfs (node * root) {
queue<node *> q;
q.push(root);
depth[root->data]=;
while (!q.empty()) {
node * now=q.front();
q.pop();
if (now->left) {q.push(now->left);depth[now->left->data]=depth[now->data]+;}
if (now->right) {q.push(now->right);depth[now->right->data]=depth[now->data]+;}
}
}
void lca (int u,int v) {
int tu=u;
int tv=v;
while (depth[tu]<depth[tv]) {
tv=father[tv];
}
while (depth[tu]>depth[tv]) {
tu=father[tu];
}
while (tu!=tv) {
tu=father[tu];
tv=father[tv];
}
if (tu==u) printf ("%d is an ancestor of %d.\n",u,v);
else if (tu==v) printf ("%d is an ancestor of %d.\n",v,u);
else printf ("LCA of %d and %d is %d.\n",u,v,tu);
}
int main () {
scanf ("%d %d",&M,&N);
for (int i=;i<=N;i++) father[i]=i;
for (int i=;i<=N;i++) {
scanf ("%d",&in[i]);
pos[in[i]]=i;
}
for (int i=;i<=N;i++)
scanf ("%d",&pre[i]);
node * root=create(,N,,N);
bfs(root);
int u,v;
for (int i=;i<M;i++) {
scanf ("%d %d",&u,&v);
if (pos[u]==&&pos[v]==) printf ("ERROR: %d and %d are not found.\n",u,v);
else if (pos[u]==||pos[v]==) printf ("ERROR: %d is not found.\n",pos[u]==?u:v);
else lca (u,v);
}
return ;
}

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