二叉树遍历

前序遍历
A->C->D->E->F->H->G->B

void traversal(Node *node) {
  if (!node->left && node->right) {res.push_back(node); return;)
   if (node->left) traversal(node->left);
  if (node->right) traversal(node->right);
}

中序遍历
D->E->C->A->H->F->B->G

void traversal(Node *node) {
   if (node->left) traversal(node->left);
  if (!node->left && node->right) {res.push_back(node); return;)
  if (node->right) traversal(node->right);
}

后序遍历
E->D->C->->H->B->G->F->A

void traversal(Node *node) {
   if (node->left) traversal(node->left);
  if (node->right) traversal(node->right);
  if (!node->left && node->right) {res.push_back(node); return;)
}