94. Binary Tree Inorder Traversal

/**
 * 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> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        inorderTraversal(root, res);
        return res;
    }
    
    private void inorderTraversal(TreeNode root, List<Integer> res) {
        if (root == null) {
            return;
        }
        inorderTraversal(root.left, res);
        res.add(root.val);
        inorderTraversal(root.right, res);
    }
}

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        
        List<Integer> left = inorderTraversal(root.left);
        List<Integer> right = inorderTraversal(root.right);
        
        res.addAll(left);
        res.add(root.val);
        res.addAll(right);
        
        return res;
    }
}

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Deque<TreeNode> stack = new LinkedList<>();
        
        TreeNode cur = root;
        while (cur != null || !stack.isEmpty()) {
            if (cur != null) {
                stack.offerFirst(cur);
                cur = cur.left;
            } else {
                cur = stack.pollFirst();
                res.add(cur.val);
                cur = cur.right;
            }
        }
        
        return res;
    }
}