给定一个二叉树，返回它的中序 遍历。

示例:

输入: [1,null,2,3]
   1
    \
     2
    /
   3

输出: [1,3,2]
进阶: 递归算法很简单，你可以通过迭代算法完成吗？

递归

/**
 * 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) {
        List < Integer > res = new ArrayList < > ();
        helper(root, res);
        return res;
    }
    public void helper(TreeNode root, List < Integer > res) {
        if(root == null)return;
        helper(root.left, res);
        res.add(root.val);
        helper(root.right, res);
    }
}

压栈

public class Solution {
    public List < Integer > inorderTraversal(TreeNode root) {
        List < Integer > res = new ArrayList < > ();
        Stack < TreeNode > stack = new Stack < > ();
        TreeNode curr = root;
        while (curr != null || !stack.isEmpty()) {
            while (curr != null) {
                stack.push(curr);
                curr = curr.left;
            }
            curr = stack.pop();
            res.add(curr.val);
            curr = curr.right;
        }
        return res;
    }
}

morris

虽然是空间O(1),但是oj并没有测出来效果，依旧空间只超过40%

/**
 * 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) {
        List < Integer > res = new ArrayList < > ();
        TreeNode curr = root;
        TreeNode pre;
        while (curr != null) {
            if (curr.left == null) {
                res.add(curr.val);
                curr = curr.right; // move to next right node
            } else { // has a left subtree
                pre = curr.left;
                while (pre.right != null) { // find rightmost
                    pre = pre.right;
                }
                pre.right = curr; // put cur after the pre node
                TreeNode temp = curr; // store cur node
                curr = curr.left; // move cur to the top of the new tree
                temp.left = null; // original cur left be null, avoid infinite loops
            }
        }
        return res;
    }
}