二叉树层序遍历

给定一个二叉树，返回该二叉树层序遍历的结果，（从左到右，一层一层地遍历）
例如：
给定的二叉树是{3,9,20,#,#,15,7},结果是[[3],[9,20],[15,7]]

/*
 * function TreeNode(x) {
 *   this.val = x;
 *   this.left = null;
 *   this.right = null;
 * }
 */

/**
  *
  * @param root TreeNode类
  * @return int整型二维数组
  */
function levelOrder( root ) {
    if (!root) return [];

    const stack = [root];
    const result = [];
    const temp = [];
    let len = 0;

    while(len = stack.length) {
        for(let i = 0; i < len; i++) {
            const node = stack.shift();
            temp.push(node.val);
            if (node.left) stack.push(node.left);
            if (node.right) stack.push(node.right);
        }
        result.push([...temp]);
        temp.length = 0;
    }

    return result;
}
module.exports = {
    levelOrder : levelOrder
};