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;

}

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