【LeetCode】二叉树非递归遍历（Java）

参考：史上最全遍历二叉树详解

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    List<Integer> res;
    public List<Integer> postorderTraversal(TreeNode root) {
        Stack<TreeNode> s = new Stack<>();
        res = new ArrayList<>();
        if(root == null) return res;
        TreeNode cur = root, pre = null;
        while(cur != null)
        {
            pre = cur.left;
            if(pre != null)
            {
                while(pre.right != null && pre.right != cur)
                    pre = pre.right;
                if(pre.right == null)
                {
                    pre.right = cur;
                    // res.add(cur.val); //pre
                    cur = cur.left;
                    continue;
                }
                else
                {
                    pre.right = null;
                    MorrisPrint(cur.left);
                }
            }
            // else
            // {
                // res.add(cur.val); //pre
            // }
                
            // res.add(cur.val); //inorder
            cur = cur.right;
        }
        MorrisPrint(root);
        return res;
    }

    public void MorrisPrint(TreeNode root)
    {
        root = reverseList(root);
        TreeNode tmp = root;
        while(tmp != null)
        {
            res.add(tmp.val);
            tmp = tmp.right;
        }
        root = reverseList(root);
    }
    public TreeNode reverseList(TreeNode root)
    {
        TreeNode pre = null;
        while(root != null)
        {
            TreeNode next = root.right;
            root.right = pre;
            pre = root;
            root = next;
        }
        return pre;
    }
}