98. Validate Binary Search Tree

Author: manahiliqbal

validateBinarySearchTree.cpp

@@ -0,0 +1,60 @@
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+ */
+class Solution {
+public:
+    bool isValidBST(TreeNode* root){
+        stack <TreeNode*> st;
+        TreeNode* prev = NULL;
+
+        while (root || !st.empty()){
+            while (root){
+                st.push(root);
+                root = root->left;
+            }
+            root = st.top();
+            st.pop();
+
+            if(prev && root->val<=prev->val){
+                return false;
+            }
+
+            prev = root;
+            root = root->right;
+        }
+        return true;
+    }
+};
+
+
+
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+ */
+class Solution {
+public:
+    bool isValidBST(TreeNode* root){
+        return isValidBSTHelper(root, LLONG_MIN, LLONG_MAX);
+    }
+    bool isValidBSTHelper(TreeNode* root, long min, long max){
+        if (root == NULL) return true;
+        if (root->val<=min || root->val>=max) return false;
+        return isValidBSTHelper(root->left, min, root->val) && isValidBSTHelper(root->right, root->val, max);
+    }
+};