生成树

在leetcode上通过给定的数组，如何生成二叉树

function generateTree(arr) {
  const tree = [[{val: arr[0]}]];
  for(let i = 1, deep = 1; i < arr.length; deep++) {
    // console.log(i)
    const before = tree[deep - 1]
    let beforeI = 0;
    const tmp = [];
    const maxJ = before.filter(it => it).length * 2;
    // console.log('maxJ',maxJ, before)
    for(let  j = 0; j < maxJ; j++) {
      while(!before[beforeI]) {
        beforeI++;
      }
      const it = arr[i + j];
      const item = (it === null || typeof it === "undefined") ? null : {val: it}
      if (j % 2 === 0) {
        if (before[beforeI]) {
          before[beforeI].left = item;
        }
      } else {
        if (before[beforeI]) {
          before[beforeI].right = item;
        }
        beforeI ++;
      }
      tmp.push(item)
    }
    i+=Math.pow(2, deep);
    tree.push(tmp)
  }
  return tree[0][0]
}

console.log(generateTree([1,2,3]))
console.log(generateTree([5,4,8,11,null,13,4,7,2,null,null,null,1]))