bfs 用队列

function TreeNode(val, left, right) {

    this.val = (val===undefined ? 0 : val)

    this.left = (left===undefined ? null : left)

    this.right = (right===undefined ? null : right)

}

---

var levelOrder = function(root) {

    let res = [];

    if(!root) return res;

    let queue = [root];

    while(queue.length){

        let size = queue.length;

        let level = [];

        for(let i=0; i<size;i++){

            let node = queue.shift();

            level.push(node.val);

            if(node.left) queue.push(node.left);

            if(node.right) queue.push(node.right);

        }

        res.push(level);

    }

    return res;

};