Given a binary tree, return theinordertraversal of its nodes' values.

For example:
Given binary tree[1,null,2,3],

   1
    \
     2
    /
   3

return[1,3,2].

tag: binary tree

recursion:

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

        ans.addAll(inorderTraversal(root.left));
        ans.add(root.val);
        ans.addAll(inorderTraversal(root.right));

        return ans;
    }
}

iterative:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> ans = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        while (root != null || !stack.isEmpty()){
            while (root != null){
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            ans.add(root.val);
            root = root.right;
        }

        return ans;
    }
}