CSES Counting Numbers
// compile: make data
// run: ./data < data.in
#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#ifdef LOCAL
#include <debug/codeforces.h>
#define debug(x...) _debug_print(#x, x);
#else
#define debug(x...) {};
#endif
template<typename...Args> void print_(Args...args){((cout<<args<<" "),...)<<endl;}
#define rep(i,a,b) for(int i=(a);i<(int)(b);++i)
#define sz(v) ((int)(v).size())
#define print(...) print_(__VA_ARGS__);
#define INTMAX (int)(9223372036854775807)
#define INF (int)(1152921504606846976)
#define double long double
#define int long long
#define MAXN 200010

int dp[20][10][3];

int count(int n) {
    if (n < 0) return 0;
    if (n == 0) return 1;
    vector<int> bit;
    while (n) bit.push_back(n % 10), n /= 10;
    reverse(bit.begin(), bit.end());
    int len = sz(bit);
    memset(dp, 0, sizeof(dp));
    rep(i, 1, 10) {
        if (i < bit[0]) dp[0][i][0] = 1;
        else if (i == bit[0]) dp[0][i][1] = 1;
        else dp[0][i][2] = 1;
    }
    rep(i, 1, len) {
        rep(lst, 0, 10) {
            rep(pre, 0, 10) {
                if (pre == lst) continue;
                dp[i][lst][0] += dp[i - 1][pre][0];
                if (lst < bit[i]) dp[i][lst][0] += dp[i - 1][pre][1];
                if (lst == bit[i]) dp[i][lst][1] += dp[i - 1][pre][1];
                dp[i][lst][2] += dp[i - 1][pre][2];
                if (lst > bit[i]) dp[i][lst][2] += dp[i - 1][pre][1];
            }
        }
    }
    int ans = 1;
    rep(i, 0, len) {
        rep(j, 0, 10) {
            if (i < len-1) ans += dp[i][j][0] + dp[i][j][1] + dp[i][j][2];
            else ans += dp[i][j][0] + dp[i][j][1];
        }
    }
    return ans;
}

int32_t main() {
    ios::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);

    int a, b; cin >> a >> b;
    cout << count(b) - count(a - 1) << endl;

    return 0;
}
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Source: github.com/k4yt3x/flowerhd
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