[Leetcode 98] Youngsang Solution

Leetcode/98/ysuh/README.md

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+## Link
+[Validate Binary Search Tree](https://leetcode.com/problems/validate-binary-search-tree/)
+
+## Topic
+- Tree
+- Recursion
+
+## Approach
+We shrink the left & right available range while traversing down the tree with recursive call
+
+## Note
+All the leaves at the left should be smaller than the parent and at the right should be greater than the parent.

Leetcode/98/ysuh/solution.cpp

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+/**
+ * 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 {
+private:
+    long int left_max_ = LONG_MAX;
+    long int right_min_ = LONG_MIN;
+    bool treeWalker(TreeNode* parent, long int left_max, long int right_min) {
+        if(parent == NULL) return true;
+        if(parent->val >= left_max || parent->val <= right_min){
+            return false;
+        }
+        return treeWalker(parent->left, parent->val, right_min) && treeWalker(parent->right, left_max ,parent->val);
+    }
+
+public:
+    bool res = true;
+    bool isValidBST(TreeNode* root) {
+        return treeWalker(root, left_max_, right_min_);
+    }
+};