leetcode 94 中序遍历二叉树

//中序遍历二叉树，解法一：递归版
//Time:O(n) SPace:O(n)
/**
 * 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:
    vector<int> inorderTraversal(TreeNode* root) 
    {
        vector<int> v;
    
        inorder(root,v);
        
        return v;
    }

    void inorder(TreeNode* root,vector<int>& v)
    {
        if(root!=NULL)
        {
             inorder(root->left,v);
             v.push_back(root->val);
             inorder(root->right,v);
        }
    }
};

//解法二：非递归版
//Time:O(n),SPace:O(n)
/**
 * 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:
    vector<int> inorderTraversal(TreeNode* root) 
    {
        vector<int> ret;
        stack<TreeNode*> s;

        while(root!=NULL||!s.empty())
        {
            while(root!=NULL)
            {
                s.push(root);
                root=root->left;
            }

            root=s.top();
            s.pop();
            ret.push_back(root->val);
            root=root->right;
        }

        return ret;
    }
};