Binary Tree Inorder Traversal

/**
 * 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> out;
    vector<int> inorderTraversal(TreeNode *root) {
        if( !root )
            return out;
        if( root->left )
        {
            inorderTraversal( root->left );
        }
        out.push_back(root->val);
        if( root->right )
        {
            inorderTraversal( root->right );
        }
        return out;
    }
};

/**非遞歸
 * 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> out;
    stack<TreeNode*> s;
    vector<int> inorderTraversal(TreeNode *root) {
        TreeNode * tmp = root;
        if( tmp == NULL )
            return out;
        while( tmp != NULL || !s.empty() )
        {
            while( tmp != NULL )
            {
                s.push(tmp);
                tmp = tmp->left;
            }
            if( !s.empty() )
            {
                tmp = s.top();
                s.pop();
                out.push_back( tmp->val );
                tmp = tmp->right;
            }
        }
        return out;
    }
};