三种方式遍历树

遍历是访问树的所有节点的过程，也可以打印它们的值。 因为所有节点都是通过边（链接）连接的，所以我们总是从根（头）节点开始。 也就是说，我们不能随机访问树中的节点。 我们使用三种方式遍历树 -

• 有序遍历
• Pre-order Traversal
• Post-order Traversal

我们现在将使用以下二叉树来查看C编程语言中树遍历的实现 -

用C实现 (Implementation in C)

#include <stdio.h>
#include <stdlib.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 <b>pre_order_traversal</b>(struct node* root) {
   if(root != NULL) {
      printf("%d ",root->data);
      pre_order_traversal(root->leftChild);
      pre_order_traversal(root->rightChild);
   }
}
void <b>inorder_traversal</b>(struct node* root) {
   if(root != NULL) {
      inorder_traversal(root->leftChild);
      printf("%d ",root->data);          
      inorder_traversal(root->rightChild);
   }
}
void <b>post_order_traversal</b>(struct node* root) {
   if(root != NULL) {
      post_order_traversal(root->leftChild);
      post_order_traversal(root->rightChild);
      printf("%d ", root->data);
   }
}
int main() {
   int i;
   int array[7] = { 27, 14, 35, 10, 19, 31, 42 };
   for(i = 0; i < 7; i++)
      insert(array[i]);
   i = 31;
   struct node * temp = search(i);
   if(temp != NULL) {
      printf("[%d] Element found.", temp->data);
      printf("\n");
   }else {
      printf("[ x ] Element not found (%d).\n", i);
   }
   i = 15;
   temp = search(i);
   if(temp != NULL) {
      printf("[%d] Element found.", temp->data);
      printf("\n");
   }else {
      printf("[ x ] Element not found (%d).\n", i);
   }            
   printf("\nPreorder traversal: ");
   pre_order_traversal(root);
   printf("\nInorder traversal: ");
   inorder_traversal(root);
   printf("\nPost order traversal: ");
   post_order_traversal(root);       
   return 0;
}

如果我们编译并运行上面的程序，它将产生以下结果 -

输出 (Output)

Visiting elements: 27 35 [31] Element found.
Visiting elements: 27 14 19 [ x ] Element not found (15).
Preorder traversal: 27 14 10 19 35 31 42 
Inorder traversal: 10 14 19 27 31 35 42 
Post order traversal: 10 19 14 31 42 35 27