Create 20 October Number of BST From Array

20 October Number of BST From Array

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+class Solution {
+  public:
+    vector<int> countBSTs(vector<int>& arr) {
+       int n = arr.size();
+        vector<int> ans;
+        vector<int> vec = arr;
+        sort(vec.begin(), vec.end());
+
+        // Precompute Catalan numbers up to n
+        vector<long long> cat(n + 1, 0);
+        cat[0] = cat[1] = 1;
+        for (int i = 2; i <= n; i++) {
+            for (int j = 0; j < i; j++) {
+                cat[i] += cat[j] * cat[i - j - 1];
+            }
+        }
+
+        for (int i = 0; i < n; i++) {
+            int idx = lower_bound(vec.begin(), vec.end(), arr[i]) - vec.begin();
+            int left = idx;              // elements smaller than arr[i]
+            int right = n - idx - 1;     // elements greater than arr[i]
+
+            long long total = cat[left] * cat[right];
+            ans.push_back((int)total);
+        }
+        return ans;
+    }
+};