[add] LeetCode 98. Validate Binary Search Tree

Author: brianway

algorithms-leetcode/README.md

@@ -58,6 +58,7 @@ Leetcode problems classified by company:
 |79|Word Search|Medium|Backtracking|待查题解|
 |94|Binary Tree Inorder Traversal|Easy|Binary Tree/Stack|一题多解，待复习|
 |96|Unique Binary Search Trees|Medium|Dynamic Programming||
+|98|Validate Binary Search Tree|Medium|Binary Search/Recursion|一题多解|
 |101|Symmetric Tree|Easy|Binary Tree/Queue/Stack|一题多解|
 |102|Binary Tree Level Order Traversal|Medium|Binary Tree,DFS/BFS|一题多解| 
 |104|Maximum Depth of Binary Tree|Easy|Binary Tree/Queue||

algorithms-leetcode/src/main/java/com/brianway/learning/algorithms/leetcode/medium/ValidateBinarySearchTree.java

@@ -0,0 +1,59 @@
+package com.brianway.learning.algorithms.leetcode.medium;
+
+import com.brianway.learning.algorithms.leetcode.common.TreeNode;
+
+/**
+ * LeetCode 98. Validate Binary Search Tree
+ * Question https://leetcode.com/problems/validate-binary-search-tree/
+ * 关键题设：无
+ *
+ * @auther brian
+ * @since 2022/9/7 22:14
+ */
+public class ValidateBinarySearchTree {
+    public boolean isValidBST(TreeNode root) {
+        return false;
+    }
+
+    /**
+     * 递归
+     * <p>
+     * 注意：不是只比较每一层的cur > left,cur < right就完了
+     * 需要 cur> any of left, cur < any of right
+     * <p>
+     * 时间复杂度：O(n)
+     * 空间复杂度：O(n)， 递归用到了栈
+     */
+    public class ValidateBinarySearchTree0 extends ValidateBinarySearchTree {
+        @Override
+        public boolean isValidBST(TreeNode root) {
+            return isValidBST(root, Long.MIN_VALUE, Long.MAX_VALUE);
+        }
+
+        /**
+         * 判断root是否在区间 (min, max) 内，左开右开
+         * 因为测试数据范围是可能到 Integer.MIN_VALUE 和 Integer.MAX_VALUE，所以这里用long
+         *
+         * @param root 当前节点
+         * @param min  下限
+         * @param max  上限
+         * @return 是否BST
+         */
+        public boolean isValidBST(TreeNode root, long min, long max) {
+            if (root == null) {
+                return true;
+            }
+
+            if (root.val <= min || root.val >= max) {
+                return false;
+            }
+
+            return isValidBST(root.left, min, root.val)
+                    && isValidBST(root.right, root.val, max);
+        }
+    }
+
+    // TODO 中序遍历，递归/迭代， 然后检查数组是否递增即可
+    // 中序遍历可以加一个pre指针，表示上一次访问的节点
+
+}