剑指 Offer 43. 1～n 整数中 1 出现的次数

233. Number of Digit One

class Solution {
public:
    int countDigitOne(int n) {
        uint32_t base = 1;
        uint32_t ans = 0;
        int N = n;
        while(n) {
            ans += n / 10 * base;
            if(n % 10) ans += base;
            if(n % 10 == 1) ans -= base - N % base - 1;
            base *= 10;
            n /= 10;
        }
        return ans;
    }
};