【题目链接】:http://codeforces.com/contest/765/problem/B

【题意】



让你把每个变量都依次替换成a,b,c,….d这些字母;

且要按顺序先用a再用b….c.d.e….z

【题解】



模拟一下这个过程就好了;

每次看看最左边那个字母是什么(当然之前已经模拟过的除外);

看看是不是当前枚举到的字母;

不是的话就错误;



【完整代码】

#include <bits/stdc++.h>

using namespace std;

#define lson l,m,rt<<1

#define rson m+1,r,rt<<1|1

#define LL long long

#define rep1(i,a,b) for (int i = a;i <= b;i++)

#define rep2(i,a,b) for (int i = a;i >= b;i--)

#define mp make_pair

#define pb push_back

#define fi first

#define se second

#define rei(x) scanf("%d",&x)

#define rel(x) scanf("%I64d",&x)

typedef pair<int,int> pii;

typedef pair<LL,LL> pll;

const int dx[9] = {0,1,-1,0,0,-1,-1,1,1};

const int dy[9] = {0,0,0,-1,1,-1,1,-1,1};

const double pi = acos(-1.0);

const int N = 500+100;

char s[N];

bool bo[N];

int len;

int main()

{

    //freopen("F:\\rush.txt","r",stdin);

    scanf("%s",s+1);

    len = strlen(s+1);

    for (char now = 'a';now <= 'z' ;now++)

    {

        int j = -1;

        rep1(i,1,len)

            if (!bo[i])

                {

                    j = i;

                    break;

                }

        if (j==-1)

            return puts("YES"),0;

        if (s[j]!=now)

            return puts("NO"),0;

        rep1(i,1,len)

            if (s[i]==now)

                bo[i] = true;

    }

    puts("YES");

    return 0;

}