Solution.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 * int val;
 * TreeNode left;
 * TreeNode right;
 * TreeNode() {}
 * TreeNode(int val) { this.val = val; }
 * TreeNode(int val, TreeNode left, TreeNode right) {
 * this.val = val;
 * this.left = left;
 * this.right = right;
 * }
 * }
 */

// Recursive Way
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        inOrder(root, result);
        return result;
    }

    public void inOrder(TreeNode root, List<Integer> result) {
        if (root == null) {
            return;
        }
        inOrder(root.left, result);
        result.add(root.val);
        inOrder(root.right, result);
    }
}

// Iterative Way

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

        }
        return result;

    }
}