Likou 94 In-order traversal of binary tree (recursive realization)

1. In-order traversal of binary tree

2. My initial thoughts and problems

Slightly, look directly at the idea and solution of force buckle

3. Problem solving method one: recursion

class Solution {
    
    
    public List<Integer> inorderTraversal(TreeNode root) {
    
    
        List<Integer> res = new ArrayList<Integer>();
        inorder(root, res);
        return res;
    }

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