255. Verify Preorder Sequence in

验证前序遍历的二叉树是否是 BST

• Runtime: 84 ms, faster than 64.71%
• Memory Usage: 43.4 MB, less than 17.65%
• 时间复杂度O(n)，空间复杂度O(n)

维护一个栈用来获取每次的根节点，遍历比较左，弹出用来比较右边

/**
 * @param {number[]} preorder
 * @return {boolean}
 */
var verifyPreorder = function(preorder) {
  let root = Number.MIN_SAFE_INTEGER;
  const stack = [];
  for (let item of preorder) {
    if (item < root) {
      return false;
    }
    let len = stack.length;
    while (len > 0 && stack[len - 1] < item) {
      root = stack.pop();
      len = stack.length;
    }
    stack.push(item);
  }
  return true;
};