Binary Tree Inorder Traversal

Given a binary tree, return the inorder traversal of its nodes’ values.

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Example
Given binary tree {1,#,2,3},

1
\
2
/
3

return [1,3,2].

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    public ArrayList<Integer> inorderTraversal(TreeNode root) {
        ArrayList<Integer> result = new ArrayList<Integer>();
        // null or leaf
        if (root == null) {
            return result;
        }

        // Divide
        ArrayList<Integer> left = inorderTraversal(root.left);
        ArrayList<Integer> right = inorderTraversal(root.right);

        // Conquer
        result.addAll(left);

        result.add(root.val);

        result.addAll(right);
        return result;
    }
}