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。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        
        Stack<TreeNode> stack = new Stack<>();
        
        List<Integer> res= new ArrayList<>();
        if(root==null)
            return res;
        
        TreeNode pnode = root;
        while(pnode!=null || !stack.isEmpty())
        {
            if(pnode!=null)
            {
                stack.push(pnode);
                pnode = pnode.left;
            }
            else
            {
                pnode = stack.pop();
                res.add(pnode.val);
                pnode = pnode.right;
            }
        }
        return res;
    }
}