102.二叉树的层序遍历

/**
 * 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 res = []
    // 利用队列实现
    let queue = []
    queue.push(root)
    while (queue.length) {
        let len = queue.length // 每一层的节点数
        let curLevel = []
        for (let i = 0; i < len; i++) {
            // 请空当前层，继续添加下一层节点
            let node = queue.shift()
            curLevel.push(node.val)
            if (node.left) {
                queue.push(node.left)
            }
            if (node.right) {
                queue.push(node.right)
            }
        }
        res.push(curLevel)
    }
    return res
};