Binary Tree Preorder Traversal

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

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