94. 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) {
        //迭代 -
        Stack<TreeNode> sta = new Stack<>();
        List<Integer> list = new ArrayList<>();
        while(root != null || !sta.isEmpty()){
            while(root != null){
                sta.push(root);
                root = root.left;
            }
            TreeNode node = sta.pop();
            list.add(node.val);
            //换为右节点
            root = node.right;            
        }
        return list;
    }
}