Morris postorder.java

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        if (root == null)
            return res;
        TreeNode sentinel = new TreeNode(0);
        sentinel.left = root;
        TreeNode curr = sentinel;
        while (curr != null) {
            if (curr.left == null) {
                curr = curr.right;
            } else {
                TreeNode temp = curr.left;
                while (temp.right != null && temp.right != curr) 
                    temp = temp.right;
                if (temp.right == null) {
                    temp.right = curr;
                    curr = curr.left;
                } else {
                    collect(res, curr.left, temp);
                    temp.right = null;
                    curr = curr.right;
                }
            }
        }
        return res;
    }
    
    private void collect(List<Integer> res, TreeNode from, TreeNode to) {
        reverse(from, to);
        TreeNode curr = to;
        while (curr != from) {
            res.add(curr.val);
            curr = curr.right;
        }
        res.add(from.val);
        reverse(to, from);
    }
    
    private void reverse(TreeNode from, TreeNode to) {
        TreeNode a = from, b = from.right;
        while (a != to) {
            TreeNode c = b.right;
            b.right = a;
            a = b;
            b = c;
        }
    }
}