How to print out a binary tree in order?

问题

I'm struggling with the printing of a binary tree that I coded. It's a little program where you can type in different states and then it orders them alphabetically in the binary tree and afterwards it should print them out again, in alphabetic order.
How should I start with the listing function? I read in the internet that a recursive algorithm would be the best, but I don't really know how to do that with my setup.

#include <iostream>
#include <stdlib.h>
#include <string>
using namespace std;


int insertInt()
{
  int x;
  cin>>x;
  while(cin.fail())
    {
      cout<<"Error! Please enter a valid integer: ";
      cin.clear();
      cin.ignore();
      cin>>x;
    }
  return x;
}//closes insertInt

struct node
{
  string info;
  node* right;
  node* left;
}; //closes node

class States
{
 private:
  node* start;

 public:
  void insert();
  void delete();
  void list();
  void search();
  States();
}; //closes States class

States::States()
{
  start = new node;
  start -> left = NULL;
  start -> right = NULL;
  start -> info = " ";
}

void States::insert()
{
  string state;
  char c;
  node *temp, *p, *s;
  p = start;
  s = start;
  temp = new node;
  temp ->info = state;

  cout<<"Please enter the state you want to add: ";
  cin>>state;

  if(s -> info == " ")
    {
      s -> info = state;
      cout<<"Added state "<<state<<"to the list.\n";
      cout<<"Ready to continue? (enter y)";
      cin>>c;
      return;
    }//close if
  else
    {
      while(true)
    {
      //moving pointer until next level is empty

      if(s->info > temp->info && s->left != NULL)
        {
          s = s->left;
          continue;
        }//close if

      if(s->info < temp->info && s->right != NULL)
        {
          s = s->right;
          continue;
        }//close if

      //inserting the new node

      if(s->info > temp->info && s-> left == NULL)
        {
          s -> left = temp;
          break;
        }//close if
      if(s->info < temp->info && s->right == NULL)
        {
          s->right = temp;
          break;
        }//cloese if

    }//close while loop
    }//close else

}//close insert function

void States::list()
{
  node *p, *s;
  p = start;
  s = start;

  if(start->info == " ")
    cout<<"Nothing to display!\n";

  if(s->left == NULL)
    cout<<"-"<<s->info<<endl;

}


void States::search()
{
  string state;
  cout<<"Please enter the state you're looking for: ";
  cin>>state;
  node *s, *temp;
  s = start;
  temp = new node;
  temp ->info = state;

  while(true)
    {
      if(s->info == state)
    {
      cout<<"Found your state!\n";
      break;
    }
      if(s->info > temp->info)
    {
      if(s->left == NULL)
        break;
      else
        s = s->left;
      continue;
    }//close if
      if(s->info < temp->info)
    {
      if(s->right == NULL)
        break;
      else
        s = s->right;
      continue;
    }//close if
    }//close while loop
}//close search function

int main()
{
  States s1;
  int c;
  int exit = 0;

  while(exit == 0)
    {
      cout<<"----------------------------------------------------------------------\n";
      cout<<"\t\tChoose one of the following options\n";
      cout<<"\t\t\t\t(1) Add a state\n";
      cout<<"\t\t\t\t(2) List states\n";
      cout<<"\t\t\t\t(3) Search for state\n";
      cout<<"\t\t\t\t(4) Delete state\n";
      cout<<"\t\t\t\t(5) Exit\n";
      cout<<"----------------------------------------------------------------------\n";
      c = insertInt();

      switch(c)
    {
    case 1:
      s1.insert();
      break;
    case 2:
      s1.list();
      break;
    case 3:
      s1.search();
      break;
    case 4:
      s1.delete();
      break;
    case 5:
      exit = 1;
      cout<<"Exiting...\n";
      break;
    default:
      cout<<"Error. Please enter a valid integer.\n";
    }//close switch
    }//closes while loop

}//closes main

回答1:

Say that node is a pointer of type Node* representing some node of your tree. Then inorder(Node*) is defined as follows:

void inorder(Node* node)
{
    if (!node) // end the recursion if node == nullptr
       return; 
    inorder(node->left);            // display the left subtree
    std::cout << node->info << " "; // display the current node
    inorder(node->right);           // display the right subtree
}

来源：https://stackoverflow.com/questions/34077497/how-to-print-out-a-binary-tree-in-order