function TreeNode(val, left = null, right = null) {
    this.val = val;
    this.left = left;
    this.right = right;
}
/**
 * 通过一个层次遍历的数组生成一棵二叉树
 * @param {any[]} array
 * @return {TreeNode}
 */
function getTreeFromLayerOrderArray(array) {
    let n = array.length;
    if (!n) return null;
    let index = 0;
    let root = new TreeNode(array[index++]);
    let queue = [root];
    while(index < n) {
        let top = queue.shift();
        let v = array[index++];
        top.left = v == null ? null : new TreeNode(v);
        if (index < n) {
            let v = array[index++];
            top.right = v == null ? null : new TreeNode(v);
        }
        if (top.left) queue.push(top.left);
        if (top.right) queue.push(top.right);
    }
    return root;
}
/**
 * 层序遍历一棵二叉树 生成一个数组
 * @param {TreeNode} root
 * @return {any[]}
 */
function getLayerOrderArrayFromTree(root) {
    let res = [];
    let que = [root];
    while (que.length) {
        let len = que.length;
        for (let i = 0; i < len; i++) {
            let cur = que.shift();
            if (cur) {
                res.push(cur.val);
                que.push(cur.left, cur.right);
            } else {
                res.push(null);
            }
        }
    }
    while (res.length > 1 && res[res.length - 1] == null) res.pop(); // 删掉结尾的 null
    return res;
}

getTreeFromLayerOrderArray([3, 4, 5, 1, 2])