Binary Tree Preorder Traversal

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

 非递归写法：

class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> preorder;
        if(!root)
            return preorder;
        stack<TreeNode*> nodes;
        nodes.push(root);
        while(!nodes.empty())
        {
            TreeNode* temp;
            temp=nodes.top();
            preorder.push_back(temp->val);
            nodes.pop();
            if(temp->right)
                nodes.push(temp->right);
            if(temp->left)
                nodes.push(temp->left);
        }
        
        return preorder;
    }
    
};