剑指 32广度优先遍历二叉树

精髓：构建队列，父弹出后必须将左右子节点push进去，每次遍历队列的头即可。

var levelOrder = function(root) {
    let res = []
    let queue = []
    queue.push(root)
    if(root===null){
        return []
    }
    while(queue.length){

      let node = queue.shift()
      //console.log(node);
      res.push(node.val)
      if (node.left!==null){
        queue.push(node.left)
      }
      if (node.right!==null){
        queue.push(node.right)
      }

    }
    return res
  };