PAT甲级 1135 - Is It A Red-Black Tree

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1. 主要是建树，可以用指针也可以用数组
2. 判断的话回溯判断两个点
   
   1. 左右孩子的当前的黑节点数目是否一样
   2. 判断当前是否为红色，则孩子节点为黑色成立与否
3. 根节点要为黑色，故最后需要确认

#include<bits/stdc++.h>
using namespace std;
const int maxn=30+2;
bool flag;
int inOrder[maxn],preOrder[maxn];
bool cmp(int a,int b){
    return abs(a)<abs(b);
}
struct node{
    int v;
    int  b_num;
}tree[1024*1024+1];
void buildTree(int cur,int preL,int preR,int inL,int inR){
    if(!flag || preL>preR || inL>inR)return;
    int i;
    for(i=inL;i<=inR;++i){
        if(preOrder[preL]==inOrder[i]){
            break;  
        }
    }
    tree[cur].v=preOrder[preL];
    buildTree(cur<<1,preL+1,preL+i-inL,inL,i-1);
    buildTree((cur<<1)|1,preR-inR+i+1,preR,i+1,inR);
    if(tree[cur<<1].b_num!=tree[(cur<<1)|1].b_num){
        flag=false;
    }
    if(tree[cur].v<0){//red
        if(tree[cur<<1].v<0 || tree[(cur<<1)|1].v<0){
            flag=false;
        }
        tree[cur].b_num=tree[cur<<1].b_num;
    }else{//black
        tree[cur].b_num=tree[cur<<1].b_num+1;
    }
}

int main(){
    int k,n;
    scanf("%d",&k);
    while(k--){
        flag=true;
        scanf("%d",&n);

        for(int i=1;i<=1024*1024;++i){
            tree[i].v=0;
            tree[i].b_num=0;
        }

        for(int i=1;i<=n;++i){
            scanf("%d",preOrder+i);
            inOrder[i]=preOrder[i];
        }
        sort(inOrder+1,inOrder+1+n,cmp);
        buildTree(1,1,n,1,n);
        if(tree[1].v<0)flag=false;
        if(flag)printf("Yes\n");
        else printf("No\n");
    }
    return 0;
}