Validate Binary Search Tree

Author: dksifoua

README.md

@@ -23,6 +23,7 @@
 | 0074 | Medium     | Search a 2D Matrix                             | Array, Binary Search, Matrix                                         | [solution](./docs/0074-Search-A-2D-Matrix.md)                             |
 | 0076 | Hard       | Minimum Window Substring                       | Hash Table, String, Sliding Window                                   | [solution](./docs/0076-Mininum-Window-Substring.md)                       |
 | 0084 | Hard       | Largest Rectangle in Histogram                 | Array, Stack, Monotonic Stack                                        | [solution](./docs/0084-Largest-Rectangle-In-Histogram.md)                 |
+| 0098 | Medium     | Validate Binary Search Tree                    | Tree, Depth-First Search, Binary Search Tree, Binary Tree            | [solution](./docs/0098-Validate-Binary-Search-Tree.md)                    |
 | 0100 | Easy       | Same Tree                                      | Tree, Depth-First Search, Breadth-First Search, Binary Tree          | [solution](./docs/0100-Same-Tree.md)                                      |
 | 0102 | Medium     | Binary Tree Level Order Traversal              | Tree, Breadth-First Search, Binary Tree                              | [solution](./docs/0102-Binary-Tree-Level-Order-Traversal.md)              |
 | 0104 | Easy       | Maximum Depth of Binary Tree                   | Tree, Depth-First Search, Breadth-First Search, Binary Tree          | [solution](./docs/0104-Maximum-Depth-of-Binary-Tree.md)                   |

docs/0098-Validate-Binary-Search-Tree.md

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+# [Validate Binary Search Tree](https://leetcode.com/problems/validate-binary-search-tree/description/)
+
+## Intuition
+
+To validate whether a binary tree is a binary search tree (BST), we need to ensure that each node satisfies the BST
+property:
+
+1. The left subtree of a node contains only nodes with keys less than the node’s key.
+2. The right subtree of a node contains only nodes with keys greater than the node’s key.
+
+The problem can be approached using a breadth-first search (BFS), where each node is checked against the valid range of
+values it can have, derived from its parent nodes.
+
+## Approach
+
+1. **Breadth-First Search (BFS):**
+    - Use a queue to traverse the tree level by level.
+    - Each element in the queue is a tuple (`Tuple3<TreeNode, Long, Long>`) that contains:
+        - The current node.
+        - The valid minimum (`min`) value the node can have.
+        - The valid maximum (`max`) value the node can have.
+    - For each node, check whether its value lies within the valid range:
+        - If `node.value` is not within `[min, max]`, the tree is invalid, so return `false`.
+        - Otherwise, update the range for the left and right children:
+            - Left child: `max` becomes the value of the current node.
+            - Right child: `min` becomes the value of the current node.
+2. **Return True for a Valid BST:** If the queue becomes empty without finding any violations of the BST property,
+   return `true`.
+
+## Complexity
+
+- **Time Complexity: `O(n)`**, where `n` is the number of nodes in the tree. Each node is processed once.
+- **Space Complexity: `O(n)`**, for the queue, which may contain up to `n` nodes in the worst case (e.g., a complete
+  binary tree).
+
+## Code
+
+- [Java](../src/main/java/io/dksifoua/leetcode/validatebinarysearchtree/Solution.java)
+
+## Summary
+
+This BFS solution ensures each node is validated against the BST property by maintaining a valid range for its values,
+derived from its parent nodes. The use of a queue and tuples makes the implementation clear and avoids recursion stack
+overhead, providing an efficient `O(n)` time complexity.

src/main/java/io/dksifoua/leetcode/utils/Tuple3.java

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+package io.dksifoua.leetcode.utils;
+
+public record Tuple3<T, U, V>(T first, U second, V third) {
+};

src/main/java/io/dksifoua/leetcode/validatebinarysearchtree/Solution.java

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+package io.dksifoua.leetcode.validatebinarysearchtree;
+
+import io.dksifoua.leetcode.utils.TreeNode;
+import io.dksifoua.leetcode.utils.Tuple3;
+
+import java.util.LinkedList;
+import java.util.Queue;
+
+public class Solution {
+
+    public boolean isValidBST(TreeNode root) {
+        if (root == null) return true;
+
+        Queue<Tuple3<TreeNode, Long, Long>> queue = new LinkedList<>() {{
+            add(new Tuple3<>(root, Long.MIN_VALUE, Long.MAX_VALUE));
+        }};
+        while (!queue.isEmpty()) {
+            Tuple3<TreeNode, Long, Long> tuple3 = queue.remove();
+            TreeNode node = tuple3.first();
+            long min = tuple3.second(), max = tuple3.third();
+
+            if (node.getValue() <= min || node.getValue() >= max) return false;
+
+            if (node.getLeft() != null) queue.add(new Tuple3<>(node.getLeft(), min, (long) node.getValue()));
+            if (node.getRight() != null) queue.add(new Tuple3<>(node.getRight(), (long) node.getValue(), max));
+        }
+
+        return true;
+    }
+}

src/test/java/io/dksifoua/leetcode/validatebinarysearchtree/SolutionTest.java

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+package io.dksifoua.leetcode.validatebinarysearchtree;
+
+import io.dksifoua.leetcode.utils.TreeNode;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class SolutionTest {
+
+    final Solution solution = new Solution();
+
+    @Test
+    void test1() {
+        assertTrue(solution.isValidBST(TreeNode.build(new Integer[] { 2, 1, 3 })));
+    }
+
+    @Test
+    void test2() {
+        assertFalse(solution.isValidBST(TreeNode.build(new Integer[] { 5, 1, 4, null, null, 3, 6 })));
+    }
+
+    @Test
+    void test3() {
+        assertTrue(solution.isValidBST(TreeNode.build(new Integer[] { 2147483647 })));
+    }
+
+    @Test
+    void test4() {
+        assertFalse(solution.isValidBST(TreeNode.build(new Integer[] { 2, 2, 2 })));
+    }
+}