43. 1～n整数中1出现的次数

class Solution {
    public int countDigitOne(int n) {
        return f(n);
    }
    
    private int f(int n){
        if(n <= 0) return 0;
        String s = String.valueOf(n);
        int h = s.charAt(0) - ‘0‘;
        //s.legnth() - 1
        int pow = (int)Math.pow(10,s.length() - 1);
        int last = n - pow * h;
        if(h == 1){
            return f(pow - 1) + last + 1 + f(last);
        }else{
            return h * f(pow - 1) + f(last) + pow;
        }
    }
}