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;
    }
};