Binary Tree Level Order Traversal

Description

Given the root of a binary tree, return the level order traversal of its nodes' values. (i.e., from left to right, level by level).

 

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: [[3],[9,20],[15,7]]

Example 2:

Input: root = [1]
Output: [[1]]

Example 3:

Input: root = []
Output: []

 

Constraints:

• The number of nodes in the tree is in the range [0, 2000].
• -1000 <= Node.val <= 1000

Solution(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) {
      let result = [];
      let queue = [root];
      while (queue[0]) {
        let queueLength = queue.length
        let level = [];
            for (let i = 0; i < queueLength; i++) {
            let current = queue.shift();
            level.push(current.val);
            if (current.left ) queue.push(current.left);
            if (current.right) queue.push(current.right);
      }
      result.push(level);
    }
    return result;
};