【codeforces 710 B Coupons and Discounts】

B. Coupons and Discounts
time limit per test1 second
memory limit per test256 megabytes
inputstandard input
outputstandard output
The programming competition season has already started and it’s time to train for ICPC. Sereja coaches his teams for a number of year and he knows that to get ready for the training session it’s not enough to prepare only problems and editorial. As the training sessions lasts for several hours, teams become hungry. Thus, Sereja orders a number of pizzas so they can eat right after the end of the competition.

Teams plan to train for n times during n consecutive days. During the training session Sereja orders exactly one pizza for each team that is present this day. He already knows that there will be ai teams on the i-th day.

There are two types of discounts in Sereja’s favourite pizzeria. The first discount works if one buys two pizzas at one day, while the second is a coupon that allows to buy one pizza during two consecutive days (two pizzas in total).

As Sereja orders really a lot of pizza at this place, he is the golden client and can use the unlimited number of discounts and coupons of any type at any days.

Sereja wants to order exactly ai pizzas on the i-th day while using only discounts and coupons. Note, that he will never buy more pizzas than he need for this particular day. Help him determine, whether he can buy the proper amount of pizzas each day if he is allowed to use only coupons and discounts. Note, that it’s also prohibited to have any active coupons after the end of the day n.

Input
The first line of input contains a single integer n (1 ≤ n ≤ 200 000) — the number of training sessions.

The second line contains n integers a1, a2, …, an (0 ≤ ai ≤ 10 000) — the number of teams that will be present on each of the days.

Output
If there is a way to order pizzas using only coupons and discounts and do not buy any extra pizzas on any of the days, then print “YES” (without quotes) in the only line of output. Otherwise, print “NO” (without quotes).

Examples
input
4
1 2 1 2
output
YES
input
3
1 0 1
output
NO
Note
In the first sample, Sereja can use one coupon to buy one pizza on the first and the second days, one coupon to buy pizza on the second and the third days and one discount to buy pizzas on the fourth days. This is the only way to order pizzas for this sample.

In the second sample, Sereja can’t use neither the coupon nor the discount without ordering an extra pizza. Note, that it’s possible that there will be no teams attending the training sessions on some days.

题意 ：两种优惠方案 ： 第一种连续两天每天买一个，第二种一次买两个，如果所有的饼都能享受优惠就输出“YES”反之输出“NO；

如果当前需要的饼数为奇数，就判断下一天需要的饼数为0者输出”NO”，反之下一天的饼数减一，同上，最后一天特判若为奇数者不能满足；

AC代码：

#include<cstdio>int pa[200010];int main(){    int N,i,kl = 0;    scanf("%d",&N);    for(i = 1;i <= N;i++)        scanf("%d",&pa[i]);    for(i = 1;i <= N; i++){        if(pa[i] % 2 && i != N ){            if(pa[i+1])                pa[i+1]--;            else                kl = 1;        }        if( i == N && pa[i] % 2)           kl = 1;        if(kl)            break;    }    if(kl)        printf("NO\n");    else        printf("YES\n");    return 0;}