README.md

Validate Binary Search Tree

Description

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.

思路

递归。带上两个参数，分别表示最小值和最大值。 左子树的最大值要小于根节点，右子树的最小值要大于根节点。

实现代码

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
  public boolean isValidBST(TreeNode root) {
    return isValidBST(root, Long.MIN_VALUE, Long.MAX_VALUE);
  }

  public boolean isValidBST(TreeNode root, long min, long max) {
    if (root == null) {
      return true;
    }

    return (min < root.val) && (root.val < max)
      && isValidBST(root.left, min, root.val)
      && isValidBST(root.right, root.val, max);
  }
}