98-validate-binary-search-tree.java

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

    public boolean dfs(TreeNode node, long lower, long upper) {
        if (node == null) {
            return true;
        }
        if (node.val <= lower || node.val >= upper) {
            return false;
        }
        return dfs(node.left, lower, node.val) && dfs(node.right, node.val, upper);
    }
}

// time O(n), n is the number of nodes (traverse)
// space O(n), n is the recursive stack size (imagine an one branch tree)
// using tree and divide and conquer and two branch top-down and bst