PAT A1151 LCA in Binary Tree

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=10010;
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+1,inR,preRoot+inRoot-inL+1,a,b);
    else if (aIn<inRoot&&bIn<inRoot)
    lca (inL,inRoot-1,preRoot+1,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=1;i<=N;i++) {
        scanf ("%d",&in[i]);
        pos[in[i]]=i;
    }
    for (int i=1;i<=N;i++)
    scanf ("%d",&pre[i]);
    int a,b;
    for (int i=0;i<M;i++) {
        scanf ("%d %d",&a,&b);
        if (pos[a]==0&&pos[b]==0) 
        printf ("ERROR: %d and %d are not found.\n",a,b);
        else if (pos[a]==0) 
        printf ("ERROR: %d is not found.\n",a);
        else if (pos[b]==0)
        printf ("ERROR: %d is not found.\n",b);
        else lca (1,N,1,a,b);
    }
    return 0;
}

 也可以根据树建图，跑lca算法

#include<bits/stdc++.h>
using namespace std;
const int maxn=10010;
struct node {
    int data;
    node * left;
    node * right;
};
int M,N;
int pre[maxn],in[maxn];
unordered_map<int,int> pos;
int father[maxn*100];
int depth[maxn*100];
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+1,preL+numLeft,inL,k-1);
    if (root->left!=NULL) father[root->left->data]=root->data;
    root->right=create(preL+numLeft+1,preR,k+1,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]=1;
    while (!q.empty()) {
        node * now=q.front();
        q.pop();
        if (now->left) {q.push(now->left);depth[now->left->data]=depth[now->data]+1;}
        if (now->right) {q.push(now->right);depth[now->right->data]=depth[now->data]+1;}
    }
}
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=1;i<=N;i++) father[i]=i;
    for (int i=1;i<=N;i++) {
        scanf ("%d",&in[i]);
        pos[in[i]]=i;
    }
    for (int i=1;i<=N;i++)
    scanf ("%d",&pre[i]);
    node * root=create(1,N,1,N);
    bfs(root);
    int u,v;
    for (int i=0;i<M;i++) {
        scanf ("%d %d",&u,&v);
        if (pos[u]==0&&pos[v]==0) printf ("ERROR: %d and %d are not found.\n",u,v);
        else if (pos[u]==0||pos[v]==0) printf ("ERROR: %d is not found.\n",pos[u]==0?u:v);
        else lca (u,v);
    }
    return 0;
}