Description：
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.
Code：
int countDigitOne(int n) { int ones = 0; for(long m = 1; m <= n; m *= 10) { long a = n / m; long b = n % m; ones += (a + 8) / 10 * m + (a % 10 == 1) * (b + 1); } return ones; }
原文：http://www.cnblogs.com/happygirl-zjj/p/4772815.html