[LeetCode] Binary Tree Inorder Traversal

/**
 * 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;
        
        TreeNode *p = root;
        stack<TreeNode*> s;
        while(p!=NULL || !s.empty()){
            
            while(p!=NULL){
                s.push(p);
                p=p->left;
            }//end while
            
            if(!s.empty()){
                p = s.top();
                s.pop();
                res.push_back(p->val) ;//刚开始定义res时，它是空的，则不能使用下标操作，只能有push_back()添加新元素
                p = p->right;   
            }//end while 
        }//end while
        return res;
    }
};