"给定二叉树 [3,9,20,null,null,15,7],\n```text\n 3\n / \\\n9 20\n / \\\n 15 7\n```\n返回其自底向上的层次遍历为：\n```text\n[\n [15,7],\n [9,20],\n [3]\n]\n```"

二叉树的层序遍历要用到迭代法，设置一个队列queue（其实就是数组），把根节点加入队列中，当队列不会空时，出队列元素，然后把这个元素的子结点加入到队列中。

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

var Tree = (function() {
    var root = null;
    var build = function(valueList) {
        if (!valueList || valueList.length === 0) {
            return
        }
        root = new TreeNode(valueList.shift())
        var queue = [root]
        var tempQueue = []
        while (queue.length > 0) {
            var node = queue.shift();
            if (valueList.length === 0) { //构建结束
                break;
            }
            var leftVal = valueList.shift()
            if (leftVal !== null) { //构建左子节点
                node.left = new TreeNode(leftVal)
                tempQueue.push(node.left)
            }

            if (valueList.length === 0) { //构建结束
                break;
            }
            var rightVal = valueList.shift()
            if (rightVal !== null) { //构建又子节点
                node.right = new TreeNode(rightVal)
                tempQueue.push(node.right)
            }
            if (queue.length === 0) { //本次构建结束
                queue = tempQueue;
            }
        }
        return this;
    }
    //层次遍历 或者广度优先遍历
    var bfsDesc = function() {
        if (!root) { return }
        var queue = [],
            result = []
        queue.push(root)
        var levelArr = [];
        var tempQueue = []
        while (queue.length > 0) {
            var node = queue.shift()
            levelArr.push(node.val)
            if (node.left) {
                tempQueue.push(node.left)
            }
            if (node.right) {
                tempQueue.push(node.right)
            }
            if (queue.length === 0) { //说明本层遍历结束
                result.push(levelArr.concat())
                levelArr = []
                queue = tempQueue
                tempQueue = []
            }
        }
        return result.reverse();
    }
    return {
        build: build,
        bfsDesc: bfsDesc
    }
})();

var tree = Tree.build([3, 9, 20, null, null, 15, 7]);
console.log(tree.bfsDesc())