[LeetCode] Validate Binary Search Tree

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

The left subtree of a node contains only nodes with keys less than the node's key.

The right subtree of a node contains only nodes with keys greater than the node's key.
Both the left and right subtrees must also be binary search trees.

这题和上题基本一样，中序遍历解决/**
* Definition for binary tree
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public boolean isValidBST(TreeNode root) {
if (root == null) {
return true;
}

List<TreeNode> pre = new ArrayList<TreeNode>();

return helper(root, pre);
}

private boolean helper(TreeNode root, List<TreeNode> pre) {
if (root == null) {
return true;
}

boolean left = helper(root.left, pre);
if (pre.size() > 0 && pre.get(0).val >= root.val) {
return false;
}

if (pre.size() < 1) {
pre.add(root);
} else {
pre.set(0, root);
}

boolean right = helper(root.right, pre);

return left && right;
}
}