validate-binary-search-tree

Author: chjung99

validate-binary-search-tree/chjung99.java

@@ -0,0 +1,59 @@
+/**
+ * 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 {
+    boolean answer = true;
+    Deque<Integer> leftStack = new ArrayDeque<>();
+    Deque<Integer> rightStack = new ArrayDeque<>();
+
+    public boolean isValidBST(TreeNode root) {
+        rightStack.push(root.val);
+        dfs(root.left, root, "left");
+        rightStack.pop();
+
+        leftStack.push(root.val);
+        dfs(root.right, root, "right");
+        leftStack.pop();
+
+        return answer;
+    }
+
+    public void dfs(TreeNode node, TreeNode parent, String direction) {
+        if (node == null) return;
+
+        if (direction.equals("left") && node.val >= parent.val){
+            answer = false;
+        }
+        if (direction.equals("right") && node.val <= parent.val){
+            answer = false;
+        }
+        if (!leftStack.isEmpty() && node.val <= leftStack.peek()) {
+            answer = false;
+        }
+        if (!rightStack.isEmpty() && node.val >= rightStack.peek()) {
+            answer = false;
+        }
+
+        rightStack.push(node.val);
+        dfs(node.left, node, "left");
+        rightStack.pop();
+
+        leftStack.push(node.val);
+        dfs(node.right, node, "right");
+        leftStack.pop();
+    }
+}
+
+