樹遍歷

前序遍歷

這是一個簡單的三個步驟。

• 存取根結點
• 遍歷左子樹
• 遍歷右子樹

void preOrder(struct node* root){
   if(root != NULL){
      printf("%d ",root->data);
      preOrder(root->leftChild);
      preOrder(root->rightChild);
   }
}

中序遍歷

這是一個簡單的三個步驟。

• 遍歷左子樹
• 存取根結點
• 遍歷右子樹

void inOrder(struct node* root){
   if(root != NULL){
      inOrder(root->leftChild);
      printf("%d ",root->data);          
      inOrder(root->rightChild);
   }
}

後序遍歷

這是一個簡單的三個步驟。

• 遍歷左子樹
• 遍歷右子樹
• 存取根結點

void postOrder(struct node* root){
   if(root != NULL){            
      postOrder(root->leftChild);
      postOrder(root->rightChild);
      printf("%d ",root->data);
   }
}

演示程式

TreeDemo.c

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

struct node {
   int data;   
   struct node *leftChild;
   struct node *rightChild;
};

struct node *root = NULL;

void insert(int data){
   struct node *tempNode = (struct node*) malloc(sizeof(struct node));
   struct node *current;
   struct node *parent;

   tempNode->data = data;
   tempNode->leftChild = NULL;
   tempNode->rightChild = NULL;

   //if tree is empty
   if(root == NULL){
      root = tempNode;
   }else{
      current = root;
      parent = NULL;

      while(1){                
         parent = current;
         //go to left of the tree
         if(data < parent->data){
            current = current->leftChild;                
            //insert to the left
            if(current == NULL){
               parent->leftChild = tempNode;
               return;
            }
         }//go to right of the tree
         else{
            current = current->rightChild;
            //insert to the right
            if(current == NULL){
               parent->rightChild = tempNode;
               return;
            }
         }
      }            
   }
}

struct node* search(int data){
   struct node *current = root;
   printf("Visiting elements: ");
	
   while(current->data != data){
      if(current != NULL)
         printf("%d ",current->data); 
			
         //go to left tree
         if(current->data > data){
            current = current->leftChild;
         }//else go to right tree
         else{                
            current = current->rightChild;
         }
			
         //not found
         if(current == NULL){
            return NULL;
         }
      }
   return current;
}

void preOrder(struct node* root){
   if(root != NULL){
      printf("%d ",root->data);
      preOrder(root->leftChild);
      preOrder(root->rightChild);
   }
}

void inOrder(struct node* root){
   if(root != NULL){
      inOrder(root->leftChild);
      printf("%d ",root->data);          
      inOrder(root->rightChild);
   }
}

void postOrder(struct node* root){
   if(root != NULL){            
      postOrder(root->leftChild);
      postOrder(root->rightChild);
      printf("%d ",root->data);
   }
}

void traverse(int traversalType){
   switch(traversalType){
      case 1:
         printf("\nPreorder traversal: ");
         preOrder(root);
         break;
      case 2:
         printf("\nInorder traversal: ");
         inOrder(root);
         break;
      case 3:
         printf("\nPostorder traversal: ");
         postOrder(root);
         break;
   }            
}  

int main() {
   /*                     11               //Level 0
   */
   insert(11); 
	
   /*                     11               //Level 0
   *                      |
   *                      |---20           //Level 1
   */
   insert(20); 
	
   /*                     11               //Level 0
   *                      |
   *                  3---|---20           //Level 1
   */
   insert(3); 
	
   /*                     11               //Level 0
   *                      |
   *                  3---|---20           //Level 1
   *                           |
   *                           |--42       //Level 2
   */
   insert(42);
	
   /*                     11               //Level 0
   *                      |
   *                  3---|---20           //Level 1
   *                           |
   *                           |--42       //Level 2
   *                               |
   *                               |--54   //Level 3
   */
   insert(54);
	
   /*                     11               //Level 0
   *                      |
   *                  3---|---20           //Level 1
   *                           |
   *                       16--|--42       //Level 2
   *                               |
   *                               |--54   //Level 3
   */
   insert(16);
	
   /*                     11               //Level 0
   *                      |
   *                  3---|---20           //Level 1
   *                           |
   *                       16--|--42       //Level 2
   *                               |
   *                           32--|--54   //Level 3
   */
   insert(32);
	
   /*                     11               //Level 0
   *                      |
   *                  3---|---20           //Level 1
   *                  |        |
   *                  |--9 16--|--42       //Level 2
   *                               |
   *                           32--|--54   //Level 3
   */
   insert(9);
	
   /*                     11               //Level 0
   *                      |
   *                  3---|---20           //Level 1
   *                  |        |
   *                  |--9 16--|--42       //Level 2
   *                     |         |
   *                  4--|     32--|--54   //Level 3
   */
   insert(4);
	
   /*                     11               //Level 0
   *                      |
   *                  3---|---20           //Level 1
   *                  |        |
   *                  |--9 16--|--42       //Level 2
   *                     |         |
   *                  4--|--10 32--|--54   //Level 3
   */
   insert(10);

   struct node * temp = search(32);
   if(temp != NULL){
      printf("Element found.\n");
      printf("( %d )",temp->data);
      printf("\n");
   }else{
      printf("Element not found.\n");
   }

   struct node *node1 = search(2);
   if(node1 != NULL){
      printf("Element found.\n");
      printf("( %d )",node1->data);
      printf("\n");
   }else{
      printf("Element not found.\n");
   }        

   //pre-order traversal
   //root, left ,right
   traverse(1);
	
   //in-order traversal
   //left, root ,right
   traverse(2);
	
   //post order traversal
   //left, right, root
   traverse(3);       
}

如果我們編譯並執行上述程式，那麼這將產生以下結果 -

Visiting elements: 11 20 42 Element found.(32)
Visiting elements: 11 3 Element not found.

Preorder traversal: 11 3 9 4 10 20 16 42 32 54 
Inorder traversal: 3 4 9 10 11 16 20 32 42 54 
Postorder traversal: 4 10 9 3 16 32 54 42 20 11