题目1509：树中两个结点的最低公共祖先

先判断两个点是否在树中，若不是则直接就找不到

若在树中，则DFS搜索连个点所在的路径，搜到了两个路径，在找两个路径最开始的相同点，也就是最近祖先结点

PS:吐槽，题目数据中结点会有相同的情况，所以用前驱寻找时会出现死循环！注意先

#include<stdio.h>
#include<stack>
#include<iostream>
using namespace std;

struct TREE{
    int v;
    TREE *left,*right;
}*rhead;

int shu[10009],all=0;
void bulid(TREE *head){
    int temp;
    scanf("%d",&temp);
    if(temp==0){
        head->left=NULL;
        head->right=NULL;
        head->v=0;
        return;
    }else{
        shu[temp]=1;
        head->v=temp;
        head->left=new TREE;
        bulid(head->left);
        head->right=new TREE;
        bulid(head->right);
    }
}

int Apre[10099],Bpre[10999];
int fa,fb,oka,okb;
void finda(TREE *head){
    
    if(oka==1)return;
    if(head->v==fa){
        oka=1;
        return;
    }
    
    if(head->left->v!=0){
        if(head->left->v!=head->v)
        Apre[head->left->v]=head->v;
        finda(head->left);
    }
    if(head->right->v!=0){
        if(head->right->v!=head->v)
        Apre[head->right->v]=head->v;
        finda(head->right);
    }
}

void findb(TREE *head){
    
    if(okb==1)return;
    if(head->v==fb){
        okb=1;
        return;
    }
    if(head->left->v!=0){
        if(head->left->v!=head->v)
        Bpre[head->left->v]=head->v;
        findb(head->left);
    }
    if(head->right->v!=0){
        if(head->right->v!=head->v)
        Bpre[head->right->v]=head->v;
        findb(head->right);
    }
}

void freeTree(TREE *head ){
    if(head->left!=NULL) freeTree(head->left);
    if(head->right!=NULL) freeTree(head->right);
    delete head;
    head=NULL;
}

int main(){
    int t;
    while(scanf("%d",&t)!=EOF)
    while(t--){
        int i;
        stack<int>sta,stb;

        for(i=0;i<=10000;i++){
            shu[i]=0;
            Apre[i]=-1;
            Bpre[i]=-1;
        }

        rhead=new TREE;
        TREE *head=rhead;
        bulid(head);
        oka=0;okb=0;
        scanf("%d%d",&fa,&fb);
        if(shu[fa]==0||shu[fb]==0){
            printf("My God\n");
        }else{
            finda(rhead);
            findb(rhead);

            int p=fa;
            sta.push(p);
            while(Apre[p]!=-1){
                p=Apre[p];
                sta.push(p);
                
            //    if(Apre[p]==p)break;
            }

            p=fb;
            stb.push(p);
            while(Bpre[p]!=-1){
                p=Bpre[p];
                stb.push(p);
                
            //    if(Bpre[p]==p)break;
            }

            int root=-1;
            while((!sta.empty())&&(!stb.empty())&&(sta.top()==stb.top())){
                root=sta.top();
                sta.pop();
                stb.pop();
            }
            if(root==-1)printf("My God\n");
            else 
            printf("%d\n",root);
            
            freeTree(rhead);
        }
    }

    return 0;
}