August 21, 2019

98. Validate Binary Search Tree

Solution: Recursive Traversal with Valid Range

找每个 node 的合理区间。

Time complexity: O(n) since we visit each node exactly once.
Space complexity: O(n) since we keep up to the entire tree.

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