94. Binary Tree Inorder Traversal

题目链接：https://leetcode.com/problems/binary-tree-inorder-traversal/

解题思路：

二叉树的中序遍历。

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 class Solution {
11     public List<Integer> inorderTraversal(TreeNode root) {
12         
13         Stack<TreeNode> stack = new Stack<>();
14         
15         List<Integer> res= new ArrayList<>();
16         if(root==null)
17             return res;
18         
19         TreeNode pnode = root;
20         while(pnode!=null || !stack.isEmpty())
21         {
22             if(pnode!=null)
23             {
24                 stack.push(pnode);
25                 pnode = pnode.left;
26             }
27             else
28             {
29                 pnode = stack.pop();
30                 res.add(pnode.val);
31                 pnode = pnode.right;
32             }
33         }
34         return res;
35     }
36 }