力扣----6. 二叉树的层序遍历(JavaScript, Java实现)

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[][]}
 */
var levelOrder = function(root) {
    if(!root) return [];
    let queue = [root];
    let res = [];
    while(queue.length){
        let temp = []
        let queueLength = queue.length;
        while(queueLength) {
            let node = queue.shift();
            node.left && queue.push(node.left);
            node.right && queue.push(node.right)
            queueLength--;
            temp.push(node.val);
        }
        res.push(temp);
    }
    return res
};