postorderTraversal = function(root) {
    
    const dfs = (node, pot) => {
        if (!node) return pot;
        dfs(node.left, pot);
        dfs(node.right, pot);
        pot.push(node.val);
        return pot;
    }
    return dfs(root, []);
}
Post-order-traversal
Time O(n) | Space O(n)
https://leetcode.com/problems/binary-tree-postorder-traversal/
@param {TreeNode} root
@return {number[]}