#251 Validate Binary Search Tree

Author: forest000014

validate-binary-search-tree/forest000014.java

@@ -0,0 +1,43 @@
+/*
+# Time Complexity: O(n)
+# Space Complexity: O(n)
+
+BST는 inorder traverse를 하면 오름차순으로 방문해야 함을 활용
+*/
+
+/**
+ * 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) {
+        List<Integer> inorder = new ArrayList<>();
+
+        inorderTraverse(root, inorder);
+
+        for (int i = 1; i < inorder.size(); i++) {
+            if (inorder.get(i - 1) >= inorder.get(i)) return false;
+        }
+
+        return true;
+    }
+
+    private void inorderTraverse(TreeNode root, List<Integer> inorder) {
+        if (root == null) return;
+
+        inorderTraverse(root.left, inorder);
+        inorder.add(root.val);
+        inorderTraverse(root.right, inorder);
+    }
+}