SOURCE

function TreeNode(val) {
    this.val = val;
    this.left = this.right = null;
}

function buildTree(arr) {
    if (!arr) return [];
    const root = new TreeNode(arr[0]);
    const q = [root];
    let index = 0;
    while (index < arr.length) {
        let cur = q.shift();
        console.log(q)
        index++;
        if (index < arr.length && arr[index]) {
            cur.left = new TreeNode(arr[index]);
            q.push(cur.left);
        }
        index++;
        if (index < arr.length && arr[index]) {
            cur.right = new TreeNode(arr[index]);
            q.push(cur.right);
        }
        console.log(cur.left)
    }
    return root;
}

function pre(root) {
    if (!root) return root;
    console.log(root.val);
    pre(root.left);
    pre(root.right);
}

function mid(root) {
    if (!root) return root;
    mid(root.left);
    console.log(root.val);
    mid(root.right);
}

const root = buildTree([1, 2, 3, 4, null, null, 7]);
pre(root);
mid(root);