数位DP模板

ll dp[21][2];
ll dfs(int pos,int limit,int lead){
    if(pos==0) return 1;
    if(!limit && dp[pos][lead]!=-1)
        return dp[pos][lead];
    ll ans=0;
    int maxx=limit?a[pos]:9;
    for(int i=0;i<=maxx;i++){
           ans+=dfs(pos-1,limit&&i==maxx,lead&&i==0);
    }
    if(!limit) dp[pos][lead]=ans;
    return ans;
}
ll solve(ll x){
    len=0;
    while(x){
        a[++len]=x%10;
        x/=10;
    }
    return dfs(len,1,1);
}
int main () {
    memset(dp,-1,sizeof dp);//init
    //.....
}