PAT-A 1143 Lowest Common Ancestor（30 分）

#include <iostream>
#include <cstdio>
#include <algorithm>

using namespace std;

#define MAXN 10005
int pre[MAXN];
int in[MAXN];

struct G{
    int value;
    G *left=NULL,*right=NULL;
};

G* build(G *root,int prel,int prer,int inl,int inr){
    int i, left, right;
    for (i = inl; i <= inr; i++)
    {
        if (in[i] == pre[prel]) break;
    }
    if (!root){
        root = new G;
        root->value=in[i];
        root->left=root->right=NULL;
    }
    if (i != inr)
    {
        root->right = build(root->right, prel+1+i-1-inl+1, prer, i+1, inr);
    }
    if (i != inl)
    {
        root->left = build(root->left, prel+1, prel+1+i-1-inl, inl, i-1);
    }
    return root;

}
bool findG(G *head,int t){
    if(!head){
        return false;
    }else{
        if(t<head->value){
            return findG(head->left,t);
        }else if(t==head->value){
            return true;
        }else{
            return findG(head->right,t);
        }
    }
}

int LCA(G *head,int a,int b){
    if(a>b){
        int t=a;
        a=b;
        b=t;
    }
    if(a<=head->value&&head->value<=b){
        return head->value;
    }else if(a<head->value&&b<head->value){
        return LCA(head->left,a,b);
    }else{
        return LCA(head->right,a,b);
    }

}

int main()
{
    int M,N,t;
    G *head=NULL;
    scanf("%d %d",&M,&N);
    for(int i=0;i<N;i++){
        scanf("%d",&t);
        pre[i]=t;
        in[i]=t;
    }
    sort(pre,pre+N);
    head=build(head,0,N-1,0,N-1);
    while(M--){
        int a,b;
        scanf("%d %d",&a,&b);
        int as,bs;
        as=findG(head,a);
        bs=findG(head,b);
        if(!as&&!bs){
            printf("ERROR: %d and %d are not found.\n",a,b);
        }else if(!as){
            printf("ERROR: %d is not found.\n",a);
        }else if(!bs){
            printf("ERROR: %d is not found.\n",b);
        }else{
            int ans=LCA(head,a,b);
            if(ans==a){
                printf("%d is an ancestor of %d.\n",a,b);
            }else if(ans==b){
                printf("%d is an ancestor of %d.\n",b,a);
            }else{
                printf("LCA of %d and %d is %d.\n",a,b,ans);
            }
        }
    }
    return 0;
}