【codeforces】 466C Number of Ways

因为最后的值是固定的，所以三部分的和是固定的。。。求一下2/3的点的后缀，for一遍就行了。。。

#include <iostream>  #include <queue>  #include <stack>  #include <map>  #include <set>  #include <bitset>  #include <cstdio>  #include <algorithm>  #include <cstring>  #include <climits>  #include <cstdlib>#include <cmath>#include <time.h>#define maxn 500005#define maxm 40005#define eps 1e-10#define mod 1000000007#define INF 999999999#define lowbit(x) (x&(-x))#define mp mark_pair#define ls o<<1#define rs o<<1 | 1#define lson o<<1, L, mid  #define rson o<<1 | 1, mid+1, R  typedef long long LL;//typedef int LL;using namespace std;LL qpow(LL a, LL b){LL res=1,base=a;while(b){if(b%2)res=res*base;base=base*base;b/=2;}return res;}LL powmod(LL a, LL b){LL res=1,base=a;while(b){if(b%2)res=res*base%mod;base=base*base%mod;b/=2;}return res;}void scanf(int &__x){__x=0;char __ch=getchar();while(__ch==' '||__ch=='\n')__ch=getchar();while(__ch>='0'&&__ch<='9')__x=__x*10+__ch-'0',__ch = getchar();}LL gcd(LL _a, LL _b){if(!_b) return _a;else return gcd(_b, _a%_b);}// headLL num[maxn], sum[maxn];int cnt[maxn];LL n, tot;void read(void){scanf("%I64d", &n), tot = 0;for(int i = 1; i <= n; i++) scanf("%I64d", &num[i]), tot += num[i];for(int i = 1; i <= n; i++) sum[i] = sum[i-1] + num[i];}void work(void){if(sum[n] % 3) {printf("0\n");return;}LL t = sum[n] / 3, ans = 0, tt = 2 * t;for(int i = 1; i <= n; i++)if(sum[i] == tt) cnt[i] = 1;else cnt[i] = 0;cnt[n] = 0;for(int i = n-1; i >= 1; i--) cnt[i] += cnt[i+1];for(int i = 2; i <= n-1; i++) if(sum[i-1] == t) ans += cnt[i];printf("%I64d\n", ans);}int main(void){read();work();return 0;}