Codeforces1062A. A Prank（暴力）

题目链接：传送门

题目：

A. A Prank
time limit per test
1 second
memory limit per test
256 megabytes
input
standard input
output
standard output

JATC and his friend Giraffe are currently in their room, solving some problems. Giraffe has written on the board an array a1
, a2, ..., an of integers, such that 1≤a1<a2<…<an≤103

, and then went to the bathroom.

JATC decided to prank his friend by erasing some consecutive elements in the array. Since he doesn't want for the prank to go too far, he will only erase in a way, such that Giraffe can still restore the array using the information from the remaining elements. Because Giraffe has created the array, he's also aware that it's an increasing array and all the elements are integers in the range [1,103]

.

JATC wonders what is the greatest number of elements he can erase?
Input

The first line of the input contains a single integer n
(1≤n≤100

) — the number of elements in the array.

The second line of the input contains n
integers ai (1≤a1<a2<⋯<an≤103

) — the array written by Giraffe.
Output

Print a single integer — the maximum number of consecutive elements in the array that JATC can erase.

If it is impossible to erase even a single element, print 0

.
Examples
Input
Copy

6
1 3 4 5 6 9

Output
Copy

2

Input
Copy

3
998 999 1000

Output
Copy

2

Input
Copy

5
1 2 3 4 5

Output
Copy

4

Note

In the first example, JATC can erase the third and fourth elements, leaving the array [1,3,_,_,6,9]

. As you can see, there is only one way to fill in the blanks.

In the second example, JATC can erase the second and the third elements. The array will become [998,_,_]
. Because all the elements are less than or equal to 1000, the array is still can be restored. Note, that he can't erase the first 2

elements.

In the third example, JATC can erase the first 4
elements. Since all the elements are greater than or equal to 1, Giraffe can still restore the array. Note, that he can't erase the last 4 elements.

题目大意：

　　一个长度为n（1 ≤ n ≤ 100）的，在1-1000范围内的严格递增序列a[n]。

　　问最多在序列里删掉多少个元素，可以使得填回去的方法唯一（要求仍为严格递增序列）。

　　比如123，就可以删掉2变成1_3，要求填进去的数比1大比3小，那么只能填2。

思路：

　　直接跑一边数组，求最长的连续整数序列的长度就好了。但是要注意细节：

　　①：n=1时，不能删去，答案为0。。好像有很多人fst在了这个上，据说输出了负值？。

　　②：a₁ = 1，a₂ = 2时，a₁可以删去，而a₁ = 2，a₂ = 3时不能删去a₁。

　　③：a_n-1 = 999，a_n = 1000时，a_n可以删去，而a_n-1 = 998，a_n = 999时不能删去a_n。

　　对于①，初始化答案为0，不断求最大的答案就可以；

　　对于②③，不妨令a[0] = 0，a[n+1] = 1001，就可以直接求最长的连续序列长度了，答案为长度-2。

代码：

#include <bits/stdc++.h>

using namespace std;
const int MAX_N = 1e2 + 5;

int a[MAX_N];

int main()
{
    int N;
    cin >> N;
    for (int i = 1; i <= N; i++) {
        scanf("%d", a+i);
    }
    a[++N] = 1001;
    int pre = 0;
    int len = 1;
    int ans = 0;
    for (int i = 1; i <= N; i++) {
        if (a[i] == pre+1) {
            len++;
            pre++;
        }
        else {
            ans = max(ans, len-2);
            len = 1;
            pre = a[i];
        }
    }
    ans = max(ans, len-2);
    cout << ans << endl;
    return 0;
}