PAT 4-1 CheckBST[1]

Given a binary tree, you are supposed to tell if it is a binary search tree. If the answer is yes, try to find the KK-th largest key, else try to find the height of the tree.

Format of function:

int CheckBST ( BinTree T, int K );

where BinTree is defined as the following:

typedef struct TNode *BinTree;
struct TNode{
    int Key;
    BinTree Left;
    BinTree Right;
};

The function CheckBST is supposed to return the K-th largest key if T is a binary search tree; or if not, return the negative height of T (for example, if the height is 55, you must return -5?5).

Here the height of a leaf node is defined to be 1. T is not empty and all its keys are positive integers. K is positive and is never more than the total number of nodes in the tree.

Sample program of judge:

#include <stdio.h>
#include <stdlib.h>

typedef struct TNode *BinTree;
struct TNode{
    int Key;
    BinTree Left;
    BinTree Right;
};

BinTree BuildTree(); /* details omitted */
int CheckBST ( BinTree T, int K );

int main()
{
    BinTree T;
    int K, out;

    T = BuildTree();
    scanf("%d", &K);
    out = CheckBST(T, K);
    if ( out < 0 )
        printf("No.  Height = %d\n", -out);
    else
        printf("Yes.  Key = %d\n", out);

    return 0;
}
/* 你的代码将被嵌在这里 */

Sample Input 1: (for the following tree)

4

Sample Output 1:

Yes.  Key = 5

Sample Input 2: (for the following tree)

3

Sample Output 2:

No.  Height = 3

解答其实过这题一个是用广度优先求出高度，另一个是用堆栈实现中缀遍历然后检查是否为从小到大的顺序遍历，啊哈三种遍历的递归解和非递归解都很重要的啦。

int CheckBST ( BinTree T, int K ){
    int level_index[1000], tmp[1000], i, j = 0, k =0, tail = 1, top = -1;
    BinTree Q[1000], Stack[1000];

    Q[0] = T;
    level_index[j] = 0;
    for(i = 0; i < 100; i++){
        if(NULL != Q[i]->Left){
            Q[tail++] = Q[i]->Left;
        }
        if(NULL != Q[i]->Right){
            Q[tail++] = Q[i]->Right;
        }
        if(i == level_index[j]&&level_index[j] != tail - 1){
            level_index[++j] = tail - 1;
        }
        else if(i == level_index[j]&&level_index[j] == tail - 1){
            break;
        }
    }
    while(-1 != top||NULL != T){
            while(NULL != T){
                Stack[++top] = T;
                T = T->Left;
            }
            T = Stack[top];
            top--;
            tmp[k++] = T->Key;
            T = T->Right;
        }
    for(k = 0; k < i; k++){
        if(tmp[k] >= tmp[k + 1]){
            return -1 - j;
        }
    }
    return tmp[i + 1 - K];
}