PAT_A1135#Is It A Red-Black Tree

Source:

PAT A1135 Is It A Red-Black Tree (30 分)

Description:

There is a kind of balanced binary search tree named red-black tree in the data structure. It has the following 5 properties:

• (1) Every node is either red or black.
• (2) The root is black.
• (3) Every leaf (NULL) is black.
• (4) If a node is red, then both its children are black.
• (5) For each node, all simple paths from the node to descendant leaves contain the same number of black nodes.

For example, the tree in Figure 1 is a red-black tree, while the ones in Figure 2 and 3 are not.

Figure 1	Figure 2	Figure 3

For each given binary search tree, you are supposed to tell if it is a legal red-black tree.

Input Specification:

Each input file contains several test cases. The first line gives a positive integer K (≤30) which is the total number of cases. For each case, the first line gives a positive integer N (≤30), the total number of nodes in the binary tree. The second line gives the preorder traversal sequence of the tree. While all the keys in a tree are positive integers, we use negative signs to represent red nodes. All the numbers in a line are separated by a space. The sample input cases correspond to the trees shown in Figure 1, 2 and 3.

Output Specification:

For each test case, print in a line "Yes" if the given tree is a red-black tree, or "No" if not.

Sample Input:

3
9
7 -2 1 5 -4 -11 8 14 -15
9
11 -2 1 -7 5 -4 8 14 -15
8
10 -7 5 -6 8 15 -11 17

Sample Output:

Yes
No
No

Keys:

• 二叉查找树（Binary Search Tree）

Attention:

• 应该注意到，红黑树（ balanced binary search tree）并不是AVL树（self-balancing binary search tree），英文题描述题这里很容易产生误解；
• 结点数目较少，给出先序遍历时，可以采用插入建树的方法；再次提醒，数据量较大时，会超时；
• 根结点的父亲预设为负值，即为红色。这样从根结点遍历时，不必特判性质2，由性质4可以推出性质

Code:

/*
Data: 2019-06-26 15:30:16
Problem: PAT_A1135#Is It A Red-Black Tree
AC: 23:52

题目大意：
红黑树具有如下性质的平衡二叉树（非AVL树）：
1.结点非红即黑
2.根结点为黑色
3.叶子结点（空结点）为黑色
4.红色结点的孩子均为黑色
5.任意结点到叶子结点构成的简单路径所含的黑色结点数目相同
现给定一棵二叉树，判定其是否为红黑树
输入：
第一行给出，测试数K<=20
第二行给出，结点总数N<=30
第三行给出，先序遍历，键值均正，负数表示红色结点

基本思路：
建树，遍历一次二叉树
根据当前结点与父结点的关系，判断性质2,4
统计各结点左子树与右子树的黑色结点个数（类似于计算AVL树的平衡因子）
若相等，则符合性质5，并根据当前结点颜色，返回黑色结点个数
*/
#include<cstdio>
#include<cmath>
using namespace std;
struct node
{
    int data;
    node *lchild,*rchild;
};

void Insert(node *&root, int x)
{
    if(root == NULL)
    {
        root = new node;
        root->data = x;
        root->lchild = root->rchild = NULL;
    }
    else if(abs(x) < abs(root->data))
        Insert(root->lchild, x);
    else
        Insert(root->rchild, x);
}

int Travel(node *root, int father, int &ans)
{
    if(ans==0 || root==NULL)
        return 0;
    int l = Travel(root->lchild, root->data, ans);
    int r = Travel(root->rchild, root->data, ans);
    if(l!=r || (father<0 && root->data<0)){
        ans=0;
        return 0;
    }
    if(root->data<0)
        return l;
    else
        return l+1;
}

int main()
{
#ifdef ONLINE_JUDGE
#else
    freopen("Test.txt", "r", stdin);
#endif // ONLINE_JUDGE

    int n,m,x;
    scanf("%d", &m);
    while(m--)
    {
        node *root = NULL;
        scanf("%d", &n);
        for(int i=0; i<n; i++)
        {
            scanf("%d", &x);
            Insert(root, x);
        }
        int ans=1;
        Travel(root,-1,ans);
        if(ans)
            printf("Yes\n");
        else
            printf("No\n");
    }

    return 0;
}