算法：98 Validate BST

按照前序遍历得到数组，然后看是否是有序的，时间优于 5%，空间优于 10%。

var isValidBST = function(root) {
    if(root == null || (root.left == null && root.right == null)) return true;

    function flatTree(root) {
        if(root == null || root == undefined) return [];
        return [...flatTree(root.left),  root.val, ...flatTree(root.right)];
    }

    let flatedList = flatTree(root);

    for(let i = 1; i< flatedList.length; i++) {
        if(flatedList[i] <= flatedList[i - 1]) return false;
    }

    return true;
};