PAT 甲级 1151

采取先建立树的办法，然后该二叉树进行遍历，记录下每个结点的父结点和层次，由于key值在int范围内即可，故采用map容器来进行映射，并作为检验该key值是否存在的依据。

寻求LCA的过程，就是先将更深层的结点回溯到较低层次的结点，并判断是否为同一结点，若是则直接返回，否则就同时继续向更低层次回溯，直到找到LCA为止

 

#include<cstdio>

#include<map>

#include<algorithm>

using namespace std;

struct node{

    int data;

    node *lchild,*rchild;

};

int inorder[10010],preorder[10010],n,m;

map<int,int> father,layer;

node* create(int inL,int inR,int preL,int preR)

{

    if(preL>preR) return NULL;

    node *root = new node;

    root->data=preorder[preL];

    root->lchild=root->rchild=NULL;

    int k;

    for(k=inL;k<=inR;k++)

        if(inorder[k]==preorder[preL]) break;

    int num2left=k-inL;

    root->lchild=create(inL,k-1,preL+1,preL+num2left);

    root->rchild=create(k+1,inR,preL+num2left+1,preR);

    return root;

    

}

void traversal(node *root,int l)

{

    layer[root->data]=l;

    if(root->lchild!=NULL)

    {

        father[root->lchild->data]=root->data;

        traversal(root->lchild,l+1);

    }

    if(root->rchild!=NULL)

    {

        father[root->rchild->data]=root->data;

        traversal(root->rchild,l+1);

    }

}

int findfather(int a,int b)

{

    if(layer[a]>layer[b]) swap(a,b);

    while(layer[a]<layer[b])

    {

        b=father[b];

    }

    if(b==a) return a;

    while(b!=a)

    {

        a=father[a];

        b=father[b];

    }

    return a;

}

int main()

{

    scanf("%d%d",&m,&n);

    for(int i=0;i<n;i++) scanf("%d",&inorder[i]);

    for(int i=0;i<n;i++) scanf("%d",&preorder[i]);

    node *root=create(0,n-1,0,n-1);

    traversal(root,0);

    while(m--)

    {

        int a,b;

        scanf("%d%d",&a,&b);

        if(layer.find(a)==layer.end()&&layer.find(b)==layer.end())

            printf("ERROR: %d and %d are not found.\n",a,b);

        else if(layer.find(a)==layer.end())

            printf("ERROR: %d is not found.\n",a);

        else if(layer.find(b)==layer.end())

            printf("ERROR: %d is not found.\n",b);

        else

        {

            int f=findfather(a,b);

            if(f==a) printf("%d is an ancestor of %d.\n",a,b);

            else if(f==b) printf("%d is an ancestor of %d.\n",b,a);

            else printf("LCA of %d and %d is %d.\n",a,b,f);

        }

    }

    return 0;

}