Leetcode 233. Number of Digit One (Medium) (cpp)

Leetcode 233. Number of Digit One (Medium) (cpp)

Tag: Math

Difficulty: Medium

/*

233. Number of Digit One (Medium)

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.

*/
class Solution {
public:
int countDigitOne(int n) {
int res = 0;
for (long long i = 1; i <= n; i *= 10) {
res += (n / i + 8) / 10 * i + (n / i % 10 == 1) * (n % i + 1);
}
return res;
}
};