Binary Tree Inorder Traversal

递归方法：

/**
 * 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 _inorderTraversal(TreeNode* root,vector<int>& inorder){
        if(root)
        {
            _inorderTraversal(root->left,inorder);
            inorder.push_back(root->val);
            _inorderTraversal(root->right,inorder);
        }
    }
    
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> inorder;
        _inorderTraversal(root,inorder);
        return inorder;
    }
};

非递归方法：

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