Create nth-digit.cpp

Author: kamyu104

C++/nth-digit.cpp

@@ -0,0 +1,20 @@
+// Time:  O(logn)
+// Space: O(1)
+
+class Solution {
+public:
+    int findNthDigit(int n) {
+        int digit_len = 1;
+        while (n > digit_len * 9 * pow(10, digit_len - 1)) {
+            n -= digit_len  * 9 * pow(10, digit_len - 1);
+            ++digit_len;
+        }
+
+        const int num = pow(10, digit_len - 1) + (n - 1) / digit_len;
+
+        int nth_digit = num / pow(10, (digit_len - 1) - (n - 1) % digit_len);
+        nth_digit %= 10;
+
+        return nth_digit;
+    }
+};