Inorder traversal Iterative approach

InorderTraversalIterative.java

package com.interviewbytes.trees;

import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Deque;
import java.util.List;

public class InorderTraversalIterative {

    public List<Integer> inorderTraversal(TreeNode root) {

        List<Integer> list = new ArrayList<>();
        Deque<TreeNode> stack = new ArrayDeque<>();

        TreeNode current = root;

        while (current != null || !stack.isEmpty()) {
            if (current != null) {
                stack.push(current);
                current = current.left;
            } else {
                current = stack.pop();
                list.add(current.val);
                current = current.right;
            }
        }

        return list;
    }
}

TreeNode.java

package com.interviewbytes.trees;

public class TreeNode {

    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}