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;
}
}