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:
- 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。