94. 二叉树的中序遍历（非递归，莫里斯遍历）.java

/**
 * 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;
            } else {
                pre = curr.left;
                while (pre.right != null) {
                    pre = pre.right;
                }
                pre.right = curr;
                TreeNode temp = curr;
                curr = curr.left;
                temp.left = null;
            }
        }
        return res;
    }
}