const postorder = function(root) {
    if (root === null) return []
    const res = []
    function dfs(root) {
        if (root === null) return;
        for (let i = 0; i < root.children.length; i++){
            dfs(root.children[i])
        }
        res.push(root.val)
    }
    dfs(root)
    return res
}