Create the-kth-factor-of-n.cpp

Author: kamyu104

C++/the-kth-factor-of-n.cpp

@@ -0,0 +1,51 @@
+// Time:  O(sqrt(n))
+// Space: O(1)
+
+class Solution {
+public:
+    int kthFactor(int n, int k) {
+        const auto& [mid, count] = kthFactor_(n);
+        int total = 2 * count - (mid * mid == n);
+        if (k > total) {
+            return -1;
+        }
+        int result = kthFactor_(n, (k <= count) ? k : total - (k - 1)).first;
+        return (k <= count) ? result : n / result;
+    }
+
+private:
+    pair<int, int> kthFactor_(int n, int k = 0) {
+        int mid = -1;
+        for (int i = 1; i * i <= n; ++i) {
+            if (n % i) {
+                continue;
+            }
+            mid = i;
+            if (!--k) {
+                break;
+            }
+        }
+        return {mid, -k};
+    }
+};
+
+// Time:  O(sqrt(n))
+// Space: O(sqrt(n))
+class Solution2 {
+public:
+    int kthFactor(int n, int k) {
+        vector<int> result;
+        for (int i = 1; i * i <= n; ++i) {
+            if (n % i) {
+                continue;
+            }
+            if (i * i != n) {
+                result.emplace_back(i);
+            }
+            if (!--k) {
+                return i;
+            }
+        }
+        return (k > result.size()) ? -1 : n / result[result.size() - k];
+    }
+};