uva 10994 - Simple Addition(规律）

#include <stdio.h>
#define ll long long
ll ans;

ll f(ll x) {
if (x == 0)	return 0;
else if (x % 10)
return x % 10;
else
return  f(x / 10);
}

void solve(ll l, ll r) {
if (r - l < 9) {
for (int i = l; i <= r; i++)
ans += f(i);
return;
}

while (l % 10) {
ans += f(l);
l++;
}

while (r % 10) {
ans += f(r);
r--;
}
ans += 45 * (r - l) / 10;
solve(l / 10, r / 10);
}

int main () {
ll l, r;
while (scanf("%lld%lld", &l, &r), l >= 0 || r >= 0) {
ans = 0;
solve(l, r);
printf("%lld\n", ans);
}
return 0;
}