Number of Digit One

Author: gavinfish

233 Number of Digit One.py

@@ -0,0 +1,29 @@
+'''
+Given an integer n, count the total number of digit 1 appearing in all non-negative integers less than or equal to n.
+
+For example:
+Given n = 13,
+Return 6, because digit 1 occurred in the following numbers: 1, 10, 11, 12, 13.
+'''
+
+class Solution(object):
+    def countDigitOne(self, n):
+        """
+        :type n: int
+        :rtype: int
+        """
+        x, m, count = n, 1, 0
+        while x > 0:
+            lastDigit = x % 10
+            x //= 10
+            count += x * m
+            if lastDigit == 1:
+                count += n % m + 1
+            elif lastDigit > 1:
+                count += m
+            m *= 10
+        return count
+
+
+if __name__ == "__main__":
+    assert Solution().countDigitOne(13) == 6