二叉树的层序遍历

[Sunny Fu]

[Li Xiangyue]

var levelOrder = function(root) {
    if(!root) return [];
    let res = [];
    let queue = [root];

    while(queue.length){
        //记录当前层级节点数
        let len = queue.length;
        //存放每一层的节点
        let curLevel = [];
        for(let i = 0; i < len; i++){
            let node = queue.shift();
            curLevel.push(node.val);
            //存入下一层的节点
            node.left && queue.push(node.left);
            node.right && queue.push(node.right);
        }
        res.push(curLevel);
    }
    return res;
};