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.

Example 1:

    2   / \  1   3

Binary tree [2,1,3], return true.

Example 2:

    1   / \  2   3

Binary tree [1,2,3], return false.

Solution1:

  public boolean isValidBST(TreeNode root) {    return isValidBST(root, Long.MIN_VALUE, Long.MAX_VALUE);  }  private boolean isValidBST(TreeNode root, long left, long right) {    if (root == null)      return true;    if ((root.left == null || (root.left.val < root.val && root.left.val > left))        && (root.right == null || (root.right.val > root.val && root.right.val < right)))      return isValidBST(root.left, left, root.val) && isValidBST(root.right, root.val, right);    return false;  }

Solution2:

better

  public boolean isValidBST(TreeNode root) {    return isValidBST(root, Long.MIN_VALUE, Long.MAX_VALUE);  }  private boolean isValidBST(TreeNode root, long left, long right) {    if (root == null)      return true;    if (root.val <= left || root.val >= right)      return false;    return isValidBST(root.left, left, root.val) && isValidBST(root.right, root.val, right);  }