HDU 4811 Ball（思維）

題解：

先放頭和尾依次左右開始塞滿，找規律。

#include <algorithm>
#include  <iostream>
#include   <cstdlib>
#include   <cstring>
#include    <cstdio>
#include    <string>
#include    <vector>
#include    <bitset>
#include     <stack>
#include     <cmath>
#include     <deque>
#include     <queue>
#include      <list>
#include       <set>
#include       <map>
#define line printf("---------------------------\n") 
#define mem(a, b) memset(a, b, sizeof(a))
#define pi acos(-1)
using namespace std;
typedef long long ll;
const int inf = 0x3f3f3f3f;
const int mod = 1e9+7;
const int maxn = 2000+10;
 
int main(){
	ll a, b, c;
	while(~scanf("%d %d %d", &a, &b, &c)){
		ll ans = 0;
		ll cnt = 0;
		if(a >= 2){
			cnt += 2;
		}else{
			cnt += a;
		}
		if(b >= 2){
			cnt += 2;
		}else{
			cnt += b;
		}
		if(c >= 2){
			cnt += 2;
		}else{
			cnt += c;
		}
		ans += (a+b+c-cnt)*cnt;
		ans += cnt*(cnt-1)/2;
		printf("%lld\n", ans);
	}
}