Number of Digit One

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.

int countDigitOne(int n) {
    int count=0;
    long m=1;
    for(m=1;m<=n;m=m*10)
    {
        count+=(n/m+8)/10*m;
        if(n/m%10==1)
          count+=n%m+1;
    }
    return count;
}