binary-tree-preorder-traversal——前序遍历

Given a binary tree, return the preorder traversal of its nodes' values.

For example:
Given binary tree{1,#,2,3},

   1
    \
     2
    /
   3

return[1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

 /**
  * Definition for binary tree
  * struct TreeNode {
  *     int val;
  *     TreeNode *left;
  *     TreeNode *right;
  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
  * };
  */
 class Solution {
 public:
     vector<int> preorderTraversal(TreeNode *root) {
         if(root==NULL)
             return v;
         v.push_back(root->val);
         preorderTraversal(root->left);
         preorderTraversal(root->right);
         return v;
     }
     vector<int> v;
 };

非递归

 /**
  * Definition for binary tree
  * struct TreeNode {
  *     int val;
  *     TreeNode *left;
  *     TreeNode *right;
  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
  * };
  */
 class Solution {
 public:
     vector<int> preorderTraversal(TreeNode *root) {
         vector<int> v;
         if(root==NULL)
             return v;
         stack<TreeNode *> tree;
         tree.push(root);
         while(!tree.empty()){
             TreeNode *p=tree.top();
             v.push_back(p->val);
             tree.pop();
             if(p->right!=NULL)
                 tree.push(p->right);
             if(p->left!=NULL)
                 tree.push(p->left);
         }
         return v;
     }
 };