[Leetcode problem]: Binary Tree preorder traversal

Given a binary tree, returnPreorderTraversal of its nodes 'values.

For example:
Given Binary Tree{1,#,2,3},

   1         2    /   3

 

Return[1,2,3].

Note:Recursive solution is trivial, cocould You Do It iteratively?

Question

The binary tree is first traversed sequentially. The recursive idea is common. Can we use iteration?

Non-recursive approach: <using Stack>

    vector<int> preorderTraversal(TreeNode *root) {        stack<TreeNode* > st;        vector<int> vi;        vi.clear();        if(!root) return vi;                st.push(root);        while(!st.empty()){            TreeNode *tmp = st.top();            vi.push_back(tmp->val);                st.pop();            if(tmp->right) st.push(tmp->right);            if(tmp->left) st.push(tmp->left);        }        return vi;    }

Recursive thinking:

class Solution {private:    vector<int> vi;public:    vector<int> preorderTraversal(TreeNode *root) {        vi.clear();        if(!root) return vi;        preorder(root);return vi;    }    void preorder(TreeNode* root){        if(!root) return;        vi.push_back(root->val);        preorder(root->left);        preorder(root->right);    }};

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Double_win Source: http://www.cnblogs.com/double-win/p/3895822.html Disclaimer: Due to my limited level, if there is anything wrong with the expression and code in the article, please criticize and correct it ~