Solution for Validate Binary Search Tree #251

first approach

second approach

children solution

update comments

개행문자 삽입

Update comments in validate-binary-search-tree solution for clarity

Author: dohyeon2

validate-binary-search-tree/dohyeon2.java

@@ -0,0 +1,51 @@
+/**
+ * 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 {
+    // TC : O(n) Each node is visited once in the loop.
+    // SC : O(n) Memory allocated for call stack
+    // In the first attempt, I tried BFS to solve the problem.
+    // But I figured out that I needed to validate a BST recursively.
+    // So, I switched to DFS to solve the problem.
+    // At first, I compared the node's value with the children's values.
+    // And I checked that those values were in the range of min and max.
+    // Then, I simplified the approach by
+    // comparing the node's value with the min and max values (assisted by ChatGPT).
+    public boolean isValidBST(TreeNode root) {
+        return dfs(root);
+    }
+
+    // Overloading the method to set default arguments
+    private boolean dfs(TreeNode node) {
+        return dfs(node, Long.MIN_VALUE, Long.MAX_VALUE);
+    }
+
+    private boolean dfs(TreeNode node, long min, long max) {
+        // It's the end of the tree, return true.
+        if (node == null) {
+            return true;
+        }
+
+        if (node.val <= min) {
+            return false;
+        }
+        if (node.val >= max) {
+            return false;
+        }
+
+        // All the result of search must be true, if the tree is valid
+        return dfs(node.left, min, node.val) && dfs(node.right, node.val, max);
+    }
+}