CodeForces 500 A. New Year Transportation

Description

New Year is coming in Line World! In this world, there are n cells numbered by integers from 1 to n, as a 1 × n board. People live in cells. However, it was hard to move between distinct cells, because of the difficulty of escaping the cell. People wanted to meet people who live in other cells.

So, user tncks0121 has made a transportation system to move between these cells, to celebrate the New Year. First, he thought of n - 1positive integers a₁, a₂, ..., a_n - 1. For every integer i where 1 ≤ i ≤ n - 1 the condition 1 ≤ a_i ≤ n - i holds. Next, he made n - 1portals, numbered by integers from 1 to n - 1. The i-th (1 ≤ i ≤ n - 1) portal connects cell i and cell (i + a_i), and one can travel from cell i to cell (i + a_i) using the i-th portal. Unfortunately, one cannot use the portal backwards, which means one cannot move from cell(i + a_i) to cell i using the i-th portal. It is easy to see that because of condition 1 ≤ a_i ≤ n - i one can't leave the Line World using portals.

Currently, I am standing at cell 1, and I want to go to cell t. However, I don't know whether it is possible to go there. Please determine whether I can go to cell t by only using the construted transportation system.

Input

The first line contains two space-separated integers n (3 ≤ n ≤ 3 × 10⁴) and t (2 ≤ t ≤ n) — the number of cells, and the index of the cell which I want to go to.

The second line contains n - 1 space-separated integers a₁, a₂, ..., a_n - 1 (1 ≤ a_i ≤ n - i). It is guaranteed, that using the given transportation system, one cannot leave the Line World.

Output

If I can go to cell t using the transportation system, print "YES". Otherwise, print "NO".

Sample Input
Input
8 4
1 2 1 2 1 2 1
Output
YES
Input
8 5
1 2 1 2 1 1 1
Output
NO
Hint
In the first sample, the visited cells are: 1, 2, 4; so we can successfully visit the cell 4.

In the second sample, the possible cells to visit are: 1, 2, 4, 6, 7, 8; so we can't visit the cell 5, which we want to visit.

题目链接：http://codeforces.com/problemset/problem/500/A

************************************************

题意：给你n个数下标为1~n，每个位置给的a[n]就是站在这个位置接着可以走的步数，要求你判断是否可以访问到下标为t的位置。

分析：直接模拟，然后进行判断就好。

AC代码：

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<queue>
#include<algorithm>
#include<cmath>
#include<iostream>

using namespace std;
typedef long long LL;

#define INF 0x3f3f3f3f
#define N 41000
#define MAXN 100000000
#define mod 1000000007

int a[N];

int main()
{
    int n,t,i;

    while(scanf("%d%d", &n,&t) != EOF)
    {
        for(i=1; i<n; i++)
            scanf("%d", &a[i]);

        i=1;
        while(1)
        {
            i=i+a[i];
            if(i==t)
            {
                printf("YES
");
                break;
            }
            if(i>t)
            {
                printf("NO
");
                break;
            }
        }

    }
    return 0;
}