Binary Tree Level Order Traversal

Problem

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

/**
 * 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) {
    const queue = root ? [root] : [];
    const res = [];

    while (queue.length) {
        const n = queue.length;
        const list = [];
        for (i = 0; i < n; i++) {
            const node = queue.shift();
            list.push(node.val);

            if (node.left) {
                queue.push(node.left);
            }
            if (node.right) {
                queue.push(node.right);
            }
        }
        res.push(list);
    }
    return res;
};

We will implement a BFS solution. We use BFS to traverse the tree by level while keeping track of the values at each level.

Problem​

Solution​

Problem

Solution