var preorderTraversal = function(root, res = []) {
    if (!root) return [];
    res.push(root.val);
    if(root.left)  preorderTraversal(root.left, res);
    if(root.right) preorderTraversal(root.right, res);
    return res;
};

var preorderTraversal = function(root) {
    if(!root) return [];
    //We need to put a root in the stack first, because Pre-order traverse from root => left => right
    let stack = [root];
    let arr = [];
    while(stack.length) {
        let curr = stack.pop();
        arr.push(curr.val);
        // we want to push right subtree is because the left subtree will be visit first then.
        if(curr.right) {
            stack.push(curr.right);
        }
        if(curr.left) {
            stack.push(curr.left);
        }
    }
    return arr;
};