C 语言中的树遍历

遍历是访问树中所有节点并打印其值的过程。由于所有节点都通过边（链接）连接，因此我们始终从根节点（头节点）开始。也就是说，我们无法随机访问树中的节点。遍历树有三种方法 −

• 中序遍历
• 前序遍历
• 后序遍历

现在我们将使用以下二叉树 − 来研究 C 语言中树遍历的实现。

C 语言实现

#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;

   //如果树为空
   if(root == NULL) {
      root = tempNode;
   } else {
      current = root;
      parent = NULL;

      while(1) { 
         parent = current;
         
         //去树的左边
         if(data < parent->data) {
            current = current->leftChild;                
            
            //向左插入
            if(current == NULL) {
               parent->leftChild = tempNode;
               return;
            }
         }  //去树的右边
         else {
            current = current->rightChild;

            //插入到右侧
            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); 

      //转到左边的树
      if(current->data > data) {
         current = current->leftChild;
      }
      //否则转到右树
      else {                
         current = current->rightChild;
      }

      //not found
      if(current == NULL) {
         return NULL;
      }
   }
   
   return current;
}

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

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

void post_order_traversal(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("
");
   }else {
      printf("[ x ] Element not found (%d).
", i);
   }

   i = 15;
   temp = search(i);

   if(temp != NULL) {
      printf("[%d] Element found.", temp->data);
      printf("
");
   }else {
      printf("[ x ] Element not found (%d).
", i);
   }            

   printf("
Preorder traversal: ");
   pre_order_traversal(root);

   printf("
Inorder traversal: ");
   inorder_traversal(root);

   printf("
Post order traversal: ");
   post_order_traversal(root);       

   return 0;
}

如果我们编译并运行上述程序，它将产生以下结果 −

输出

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

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