Create numbers-with-repeated-digits.cpp

Author: kamyu104

C++/numbers-with-repeated-digits.cpp

@@ -0,0 +1,48 @@
+// Time:  O(logn)
+// Space: O(logn)
+
+class Solution {
+public:
+    int numDupDigitsAtMostN(int N) {
+        const auto& digits = to_string(N + 1);
+        int result = 0;
+
+        // Given 321
+        //
+        // 1. count numbers without repeated digits:
+        // - X
+        // - XX
+        for (int i = 1; i < digits.length(); ++i) {
+            result += P(9, 1) * P(9, i - 1);
+        }
+
+        // 2. count numbers without repeated digits:
+        // - 1XX ~ 3XX
+        // - 30X ~ 32X
+        // - 320 ~ 321
+        unordered_set<int> prefix_set;
+        for (int i = 0; i < digits.length(); ++i) {
+            for (int d = (i == 0) ? 1 : 0; d < digits[i] - '0'; ++d) {
+                if (prefix_set.count(d)) {
+                    continue;
+                }
+                result += P(9 - i, digits.length() - i - 1);
+            }
+            if (prefix_set.count(digits[i] - '0')) {
+                break;
+            }
+            prefix_set.emplace(digits[i] - '0');
+        }
+        return N - result;
+    }
+
+private:
+    int P(int m, int n) const {
+        int result = 1;
+        while (n > 0) {
+            result *= m - n + 1;
+            --n;
+        }
+        return result;
+    }
+};