BinaryTreeInorderTraversal.java

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Stack;

public class BinaryTreeInorderTraversal {

    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new LinkedList<Integer>();
        Stack<TreeNode> stack = new Stack<TreeNode>();
        while (!stack.isEmpty() || root != null) {
            if (root != null) {
                stack.push(root);
                root = root.left;
            } else {
                root = stack.pop();
                list.add(root.val);
                root = root.right;
            }
        }
        return list;
    }

    public List<Integer> inorderTraversal2(TreeNode root) {
        if (root == null) {
            return new ArrayList<>();
        }

        List<Integer> res = new ArrayList<>();
        TreeNode pre = null;

        while (root != null) {
            if (root.left == null) {
                res.add(root.val);
                root = root.right;
            } else {
                pre = root.left;
                while (pre.right != null && pre.right != root) {
                    pre = pre.right;
                }
                if (pre.right == null) {
                    pre.right = root;
                    root = root.left;
                } else {
                    pre.right = null;
                    res.add(root.val);
                    root = root.right;
                }
            }
        }

        return res;
    }
}