104.二叉树的最大深度

/**
 * 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 maxDepth = function (root) {
    if (!root) {
        return 0
    }
    // 使用层序遍历实现
    let count = 0
    let queue = []
    queue.push(root)
    while (queue.length) {
        let len = queue.length // 当前层的节点数
        for (let i = 0; i < len; i++) {
            // 清空当前层，添加下一次节点
            let node = queue.shift()
            if (node.left) {
                queue.push(node.left)
            }
            if (node.right) {
                queue.push(node.right)
            }
        }
        count++
    }
    return count
};