[leetcode]94. Binary Tree Inorder Traversal

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

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,3,2]

Solution:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    List<Integer> list = new ArrayList<>();
    public List<Integer> inorderTraversal(TreeNode root) {
        if(root == null) {
            return list;
        }
        if(root.left != null) {
            inorderTraversal(root.left);
        }
        
        if(root != null) {
            list.add(root.val);
        }
        if(root.right != null) {
            inorderTraversal(root.right);
        }
        
        return list;
    }
}