[Leetcode] Number of Digit Ones

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.

Hint:

1. Beware of overflow.

大神的解答，讲得很清楚：

对于每一位，都需要考虑3种情况：0，1，2。

http://blog.csdn.net/elton_xiao/article/details/48578387