[leetcode] Binary Tree Inorder Traversal

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

For example:
Given binary tree {1,#,2,3},

   1
    
     2
    /
   3

return [1,3,2].

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

OJ's Binary Tree Serialization:

The serialization of a binary tree follows a level order traversal, where '#' signifies a path terminator where no node exists below.

Here's an example:

   1
  / 
 2   3
    /
   4
    
     5

The above binary tree is serialized as "{1,2,3,#,#,4,#,#,5}".

使用非递归的方法中序遍历二叉树。

使用一个栈来模拟递归过程，如果游走指针不为NULL，就把节点入栈，继续访问其左孩子。否则的话访问栈顶元素，并把指针指向他的右孩子。访问栈顶和跳转到右孩子一定要在一个语句块中实现，原因有二，其一为从栈顶弹出的指针的左孩子一定已经访问过了，所以按中序遍历的顺序应该访问该节点，其二这样做就无需判断游走指针指向的节点的左孩子是否访问过，应为栈顶的元素的左孩子一定访问过，只需访问其右孩子即可。

代码如下：

/**
 * 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> inorderTraversal(TreeNode *root) {
            vector<int> res;
            if( root == NULL )
            {
                return res;
            }
            TreeNode * tr = root;

            stack<TreeNode*> s;
            while(s.size()>0 || tr != NULL)
            {
                if(tr!=NULL)
                {
                    s.push(tr);
                    tr= tr->left;
                }
                else
                {
                    tr = s.top();
                    s.pop();
                    res.push_back(tr->val);
                    tr = tr->right;
                }
            }
            return res;
        }
};

并附上其递归版本：

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void T(TreeNode * root,vector<int> & r)
    {
        if(root==NULL) return;
        T(root->left,r);
        r.push_back(root->val);
        T(root->right,r);
    }
    vector<int> inorderTraversal(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<int> r;
        T(root,r);
        return r;
    }
};