c++ - 当我声明 root->right->right = newnode(7);评论，编译器显示运行时错误。谁能解释为什么？

Question

我正在用 C++ 编写计算二叉树中根到叶路径总数的代码。

#include<stdio.h>
#include<iostream>
#include<stdlib.h>
#include<queue>
using namespace std;  
struct node  
{int data;  
 struct node *left;  
 struct node *right;  
};

struct node* newnode(int data)  
{struct node* node = (struct node*)malloc(sizeof(struct node));  
 node->data = data;  
 node->left = NULL;  
 node->right = NULL;  
 return node;  
}

int root_to_leaf_paths(struct node *root)  
{if(root->left == NULL && root->right == NULL)  
  return 1;  
 else if(root == NULL)  
   return 0;  
 else  
   return(root_to_leaf_paths(root->left)+root_to_leaf_paths(root->right));  

}

int main()  
{
 struct node *root = newnode(1);  
 root->left = newnode(2);  
 root->right = newnode(3);  
 root->left->left = newnode(4);  
 root->left->right = newnode(5);  
 root->right->left = newnode(6); 

当我添加以下行注释时，编译器显示运行时错误。

 **//root->right->right = newnode(7);**  

printf("%d",root_to_leaf_paths(root));  
}

Answer 1

条件的顺序是倒序的:

 if(root->left == NULL && root->right == NULL)  
  return 1;  
 else if(root == NULL)  
   return 0;  
 else  
   return(root_to_leaf_paths(root->left)+root_to_leaf_paths(root->right));  

在访问root->left 等之前，您需要先确定root 不为空