[leetcode] Validate Binary Search Tree dfs

方法1: inorder traversal, 如果in order就是valid (用int[] 做publicly accessible place，就不能过min_value, 这样的话就要用arraylist，然后放null）

方法2: 按definition构建recursion, left: helper(root.left, min, root.val);
right: helper(root.right, root.val, max)

1. 

public boolean isValidBST(TreeNode root) {
ArrayList<Integer> pre = new ArrayList<Integer>();
pre.add(null);
return helper(root, pre);
}

private boolean helper(TreeNode root, List<Integer> pre) {
if (root==null) return true;
boolean left = helper(root.left, pre);
if (!left) return false;
if (pre.get(0)!=null && root.val<=pre.get(0)) return false;
pre.set(0, root.val);
return helper(root.right, pre);
}

2: ( 过不了oj，因为有min_value test case)

public boolean isValidBST(TreeNode root) {
return helper(root, Integer.MIN_VALUE, Integer.MAX_VALUE);
}
boolean helper(TreeNode root, int min, int max)
{
if(root == null)
return true;
if(root.val <= min || root.val >= max)
return false;
return helper(root.left, min, root.val) && helper(root.right, root.val, max);
}