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

For example:

Given binary tree [1,null,2,3],

   1
    \
     2
    /
   3

return [1,3,2].

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

C++ Solution:

/**

  * 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> result;

       
        stack<TreeNode*> stack;

        TreeNode* curr = root;

        while (curr || !stack.empty()) {

            while (curr) {

                stack.push(curr);

                curr = curr->left;

            }

            curr = stack.top();

             stack.pop();

            result.push_back(curr->val);

            curr = curr->right;

        }

        return result;

    }

   
    void traversal(TreeNode *root, vector<int> &list) {

        if (root != NULL) {

            if (root->left != NULL)

                 traversal(root->left, list);

            list.push_back(root->val);

            if (root->right != NULL)

                 traversal(root->right, list);

        }

    }

};