Leetcode 94. Binary Tree Inorder Traversal

Stack.

public class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> ret = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<TreeNode>();
        
        while (root != null || !stack.empty()) {
            while (root != null) {
                stack.add(root);
                root = root.left;
            }
            root = stack.pop();
            ret.add(root.val);
            root = root.right;
        }
        
        return ret;
    }
}

Recursive.

public class Solution {
    public static void inorder(TreeNode root, List<Integer> list) {
        if (root == null) return;
        inorder(root.left, list);
        list.add(root.val);
        inorder(root.right, list);
    }
    
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        inorder(root, list);
        return list;
    }
}