var isValidBST = function(root) {
let flag = true
const check = (root, max, min) => {
if(!root || !flag) return
if(root.val <= max || root.val >= min) {
flag = false
return
}
check(root.left, max, root.val)
if(!flag) return
check(root.right, root.val, min)
}
check(root, -Infinity , Infinity)
return flag
};