
# Binary Tree Level Order Traversal

## Description

[doc](https://leetcode.com/problems/binary-tree-level-order-traversal/)

## Solutions

### First Idea

```javascript
/**
 * 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 []
  }

  let result = []
  let curLevel = []

  let curQueue = [root]
  let nextQueue = []

  while (curQueue.length) {
    const cur = curQueue.shift()

    curLevel.push(cur.val)
    cur.left && nextQueue.push(cur.left)
    cur.right && nextQueue.push(cur.right)

    if (!curQueue.length) {
      curQueue = nextQueue
      nextQueue = []
      result.push(curLevel)
      curLevel = []
    }
  }

  return result
}
```

- Time Complexity: O(n)
- Space Complexity: O(n)
