102. 二叉树的层序遍历

// 给你二叉树的根节点 root ，返回其节点值的 层序遍历 。 （即逐层地，从左到右访问所有节点）。
/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[][]}
 */
var levelOrder = function(root) {
  if (!root) return [];
  const res = [];
  const stack = [root];
  while (stack.length) {
    let length = stack.length;

    if (res.length === 0) res[0] = [];
    else res[res.length] = [];

    while (length--) {
      const cur = stack.shift();
      res[res.length - 1].push(cur.val);
      cur.left && stack.push(cur.left);
      cur.right && stack.push(cur.right);
    }
  }
  return res;
};

const tree = {
  val: 3,
  left: {
    val: 9,
  },
  right: {
    val: 20,
    left: {
      val: 15,
    },
    right: {
      val: 7,
    },
  },
};

console.log(levelOrder(tree)); // 2