题目描述

很基本的一个数位统计dp问题

一道更通用的练习题：
AcWing 338. 计数问题

C++ 代码

#include <vector>
#include <iostream>
using namespace std;
const int N = 10;
int get(vector<int> num, int l, int r)
{
    int res = 0;
    for (int i = l; i >= r; i -- ) res = res * 10 + num[i];
    return res;
}
int power10(int x)
{
    int res = 1;
    while (x -- ) res *= 10;
    return res;
}
int count(int n, int x)
{
    if (!n) return 0;
    vector<int> num;
    while (n)
    {
        num.push_back(n % 10);
        n /= 10;
    }
    n = num.size();
    int res = 0;
    for (int i = n - 1 - !x; i >= 0; i -- )
    {
        if (i < n - 1)
        {
            res += get(num, n - 1, i + 1) * power10(i);
            if (!x) res -= power10(i);
        }
        if (num[i] == x) res += get(num, i - 1, 0) + 1;
        else if (num[i] > x) res += power10(i);
    }
    return res;
}
int main()
{
    int a;
    cin >> a;
    cout << count(a, 1);
    return 0;
}