Binary Tree Inorder Traversal

Given a binary tree, return the inordertraversal of its nodes' values.

Example:

Input: [1,null,2,3]
   1
    
     2
    /
   3

Output: [1,3,2]

Follow up: Recursive solution is trivial, could you do it iteratively?

typedef char ElemType;

typedef struct BiTreeNode
{
    ElemType data;
    struct BiTreeNode *left;
    struct BiTreeNode *right;
}BiTreeNode,*BiTree;

Binary Tree Inorder Traversal
void InOrderBiTree(BiTree T){
    if(T == NULL)
        return;
        
   InOrderBiTree(T->left);
   printf("%c",T->data);
   InOrderBiTree(T->right);
}

以下方法可作参考，优先学习以下两种解法：

//recursion
C++ Solution1:
class Solution{
    public:
        vector<int> inorderTraversal(TreeNode *root){
            vector<int> res;
            inorder(root,res);
            return res;
        }
    
    void inorder(TreeNode *root, vector<int> &res){
        if(!root) return;
        if(root->left) inorder(root->left,res);
        res.push_back(root->val);
        if(root->right) inorder(root->right,res);
    }
};

//no recursion:
class Solution{
    public:
        vector<int> inorderTraversal(TreeNode *root){
            vector<int> res;
            stack<TreeNode*> s;
            TreeNode *p = root;
            while(!s.empty() || p){
                if(p){
                    s.push(p);
                    p = p->left;
                }
                else{
                    TreeNode *t = s.top();s.pop();
                    res.push_back(t->val);
                    p = t->right;   
                }
            }
            return res;
        }
};