Numbers of Length N solution

Math/NumbersOfLengthN.cpp

@@ -0,0 +1,50 @@
+int all(bool digits[], int B, int total) {
+    bool isZero = digits[0] ? 1 : 0;
+    long long int count = 1;
+    if (B == 1) {
+        return total;
+    }
+    count *= isZero ? total - 1 : total;
+    count *= pow(total, B - 1);
+    return count;
+}
+int compare(bool digits[], int B, int C, int total) {
+    string c = to_string(C);
+    vector<int> count(10);
+    for (int i = 1; i < 10; i ++) {
+        count[i] = digits[i - 1] + count[i - 1];
+    }
+    int dp[B + 1];
+    dp[0] = 0;
+    bool flag = 1;
+    for (int i = 1; i <= B; i ++) {
+        dp[i] = dp[i - 1] * total;
+        if (flag || i == 1) {
+            int smallCount = count[c[i - 1] - '0'];
+            if (B != 1 && i == 1 && digits[0]) {
+                smallCount --;
+            }
+            dp[i] += smallCount;
+            if (!digits[c[i - 1] - '0']) {
+                flag = false;
+            }
+        }
+    }
+    return dp[B];
+}
+int Solution::solve(vector<int> &A, int B, int C) {
+    int length = log10(C) + 1;
+    bool digits[10] = {0};
+    int total = 0;
+    for (int i = 0; i < A.size(); i ++) {
+        digits[A[i]] = 1;
+        total ++;
+    }
+    if (B > length || B == 0) {
+        return 0;
+    } else if (B < length) {
+        return all(digits, B, total);
+    } else {
+        return compare(digits, B, C, total);    
+    }
+}