浙大PAT 1043题 1043. Is It a Binary Search Tree

这题很久之前敲过一次，现在话一1个小时又敲了一次，感觉还是这次的代码简洁优美。

之前的代码：

#include<stdio.h>#include<stdlib.h>typedef struct Node{  int value;  struct Node* left;  struct Node* right;}Node;int num[1008];int flag;void BuildBST(Node* node,int start,int end){  if(flag==0) return;  int middle=start+1;  while(num[middle]<num[start]&&middle<end){    middle++;  }  int tmp=middle;  while(num[middle]>=num[start]&&middle<end){    middle++;  }  if(middle!=end){    flag=0;    node->left=NULL;    node->right=NULL;    return;  }  node->value=num[start];  if(tmp==start+1){    node->left=NULL;  }  else{    node->left=(Node*)malloc(sizeof(Node));    BuildBST(node->left,start+1,tmp);  }  if(tmp==end){    node->right=NULL;  }  else{    node->right=(Node*)malloc(sizeof(Node));    BuildBST(node->right,tmp,end);  }  }void BuildMirrorBST(Node* node,int start,int end){  if(flag==0) return;  int middle=start+1;  while(num[middle]>=num[start]&&middle<end){    middle++;  }  int tmp=middle;  while(num[middle]<num[start]&&middle<end){    middle++;  }  if(middle!=end){    flag=0;    node->left=NULL;    node->right=NULL;    return;  }  node->value=num[start];  if(tmp==start+1){    node->left=NULL;  }  else{    node->left=(Node*)malloc(sizeof(Node));    BuildMirrorBST(node->left,start+1,tmp);  }  if(tmp==end){    node->right=NULL;  }  else{    node->right=(Node*)malloc(sizeof(Node));    BuildMirrorBST(node->right,tmp,end);  }  }void PrintPostorderTree(Node* node){  if(node==NULL) return;  PrintPostorderTree(node->left);  PrintPostorderTree(node->right);  printf("%d ",node->value);}void PrintPostorderRoot(Node* root){  PrintPostorderTree(root->left);  PrintPostorderTree(root->right);  printf("%d\n",root->value);}void dealloc(Node* node){     if(node==NULL) return;     dealloc(node->left);     dealloc(node->right);     free(node);   }int main(){  int i,n;  scanf("%d",&n);  for(i=0;i<n;i++){    scanf("%d",&num[i]);  }  flag=1;  Node* root=(Node*)malloc(sizeof(Node));  BuildBST(root,0,n);  if(flag){    printf("YES\n");    PrintPostorderRoot(root);    dealloc(root);  }  else{    flag=1;    dealloc(root->left);    dealloc(root->right);    BuildMirrorBST(root,0,n);    if(flag){      printf("YES\n");      PrintPostorderRoot(root);    }    else{      printf("NO\n");    }    dealloc(root);  }  return 0;}

这次的代码，还是这次的建树方法好：

#include<stdio.h>#include<stdlib.h>int arr[1005],flag;typedef struct NodeTpye{int value;struct NodeTpye* left;struct NodeTpye* right;}Node;Node* buildBST(int s,int e){int i,j;if(s>e) return NULL;for(i=s+1;i<=e;i++){if(arr[i]>=arr[s]){break;}}for(j=i+1;j<=e;j++){if(arr[j]<arr[s]){flag=0;return NULL;}}Node* node=(Node*)malloc(sizeof(Node));node->value=arr[s];node->left=buildBST(s+1,i-1);node->right=buildBST(i,e);return node;}Node* buildMirrorBST(int s,int e){int i,j;if(s>e) return NULL;for(i=s+1;i<=e;i++){if(arr[i]<arr[s]){break;}}for(j=i+1;j<=e;j++){if(arr[j]>=arr[s]){flag=0;return NULL;}}Node* node=(Node*)malloc(sizeof(Node));node->value=arr[s];node->left=buildMirrorBST(s+1,i-1);node->right=buildMirrorBST(i,e);return node;}void printPostorderTree(Node* node){if(node==NULL) return;printPostorderTree(node->left);printPostorderTree(node->right);printf("%d ",node->value);}void delloc(Node* node){if(node==NULL) return;delloc(node->left);delloc(node->right);free(node);}int main(){int i,n;scanf("%d",&n);for(i=0;i<n;i++){scanf("%d",&arr[i]);}flag=1;Node* root=buildBST(0,n-1);if(flag){printf("YES\n");printPostorderTree(root->left);printPostorderTree(root->right);printf("%d\n",root->value);delloc(root);}else{delloc(root);flag=1;Node* root=buildMirrorBST(0,n-1);if(flag){printf("YES\n");printPostorderTree(root->left);printPostorderTree(root->right);printf("%d\n",root->value);delloc(root);}else{printf("NO\n");delloc(root);}}return 0;}