[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?
非递归中序遍历，基础吧， s.pop（） 原来是返回空的。。。

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        stack<TreeNode*> S;
    	vector<int> ret;
		ret.clear();
		if(root==NULL)
			return ret;
		TreeNode *p =root; 
		while(!S.empty()|| p)
		{
			if(p!=NULL)
			{
				S.push(p);
				p= p->left;
			}else{
				p = S.top();
				S.pop();
				ret.push_back(p->val);
				p = p->right;
			}
		}
		return ret;
    }
};