https://www.v2ex.com/t/334087

iteration.cpp

#include <cstddef>
#include <iostream>
#include <vector>
#include <queue>

using namespace std;

struct TreeNode {
  int val;
  TreeNode* left;
  TreeNode* right;
};


void print_tree(TreeNode* root) {
  TreeNode* node;
  vector<int> result;
  queue<TreeNode*> q;
  q.push(root);
  
  while (!q.empty()) {
    node = q.front();
    q.pop();
    result.push_back(node->val);
    if (node->right) q.push(node->right);
    if (node->left) q.push(node->left);
  }
  
  for (vector<int>::iterator it = result.end()-1; it >= result.begin(); it--) {
    cout << (*it) << ", ";
  }
}

int main() {
  TreeNode n1 = {1, nullptr, nullptr};
  TreeNode n2 = {2, nullptr, nullptr};
  TreeNode n3 = {3, nullptr, nullptr};
  TreeNode n4 = {4, nullptr, nullptr};
  TreeNode n5 = {5, nullptr, nullptr};
  TreeNode n6 = {6, nullptr, nullptr};
  TreeNode n7 = {7, nullptr, nullptr};
  TreeNode n9 = {9, nullptr, nullptr};
  TreeNode n10 = {10, nullptr, nullptr};
  n1.left = &n2;
  n1.right = &n7;
  n2.left = &n3;
  n2.right = &n4;
  n4.left = &n9;
  n4.right = &n10;
  n7.left = &n5;
  n7.right = &n6;
  print_tree(&n1);
}

resursion.cpp

#include <cstddef>
#include <iostream>
#include <list>

using namespace std;

struct TreeNode {
  int val;
  TreeNode* left;
  TreeNode* right;
};

void traverse(list<TreeNode*> queue, list<int> &result) {
  list<TreeNode*> next_q;
  TreeNode *node;
  while (!queue.empty()) {
    node = queue.front();
    queue.pop_front();
    result.push_back(node -> val);
    if (node->right) next_q.push_back(node->right);
    if (node->left) next_q.push_back(node->left);
  }
  if (!next_q.empty()) traverse(next_q, result);
}

void print_tree(TreeNode* root) {
  list<TreeNode*> queue;
  list<int> result;
  queue.push_back(root);
  traverse(queue, result);

  for (list<int>::reverse_iterator it = result.rbegin(); it != result.rend(); it++) {
    cout << (*it) << ", ";
  }
}

int main() {
  TreeNode n1 = {1, nullptr, nullptr};
  TreeNode n2 = {2, nullptr, nullptr};
  TreeNode n3 = {3, nullptr, nullptr};
  TreeNode n4 = {4, nullptr, nullptr};
  TreeNode n5 = {5, nullptr, nullptr};
  TreeNode n6 = {6, nullptr, nullptr};
  TreeNode n7 = {7, nullptr, nullptr};
  TreeNode n9 = {9, nullptr, nullptr};
  TreeNode n10 = {10, nullptr, nullptr};
  n1.left = &n2;
  n1.right = &n7;
  n2.left = &n3;
  n2.right = &n4;
  n4.left = &n9;
  n4.right = &n10;
  n7.left = &n5;
  n7.right = &n6;
  print_tree(&n1);
}