C代码给出不同的输出
问题描述:
我试图解决Code Forces中的问题 - http://codeforces.com/problemset/problem/680/B。我已经在本地解决了它,但是当我将它上传到Code Forces时,它会提供不同的输出。C代码给出不同的输出
目前,这是我的代码:
#include <stdio.h>
int main()
{
int q, pos;
scanf("%i %i", &q, &pos);
int cities[q];
int criminal_count = 0;
//the greatest distance is naturally the number of cities
int criminals_by_dist[q];
for (int i = 0; i < q; ++i)
criminals_by_dist[i] = 0;
for (int i = 0; i < q; ++i)
scanf("%i", &cities[i]);
//now we have the cites, lets count
//first the centre
if (cities[pos - 1] > 0)
criminals_by_dist[0]++;
int l = 0, r = 0;
for (int i = 1; i < q; ++i)
{
//count how many on the left of current position
//first check if it is not out of range
l = pos - i;
if (l >= 0)
criminals_by_dist[i] += cities[l - 1];
//same with the right
//first check if it is not out of range
r = pos + i;
if (r < q)
criminals_by_dist[i] += cities[r - 1];
}
//count how many criminals can be secured in a particular city
//the centre is always confirmed because there is only one centre
criminal_count += criminals_by_dist[0];
int current = 0;
for (int i = 1; i < q; ++i)
{
current = criminals_by_dist[i];
if ((current == 2 || (pos - i - 1 >= 0 != pos + i - 1 < q)))
criminal_count += current;
}
printf("%i", criminal_count);
return 0;
}
在我的控制台,我输入以下输入:
6 3
1 1 1 0 1 0
,输出是:
3
然而, codeforces,会发生以下情况:
输入
6 3
1 1 1 0 1 0
输出
1998776724
回答
3
它是所有相同的代码。为什么会发生?
答
你的算法不太正确。
在这条线
l = pos - i;
l
成为在某些时候比1小,因此您可以访问城市越界这是不确定的行为。
修改如下:
#include <assert.h>
...
//count how many on the left of current position
//first check if it is not out of range
l = pos - i;
assert(l > 0); // <<< add this line
if (l >= 0)
criminals_by_dist[i] += cities[l - 1];
再次运行程序,你会看看会发生什么。
请解释你的代码应该做什么。 – Yunnosch
'if(l> = 0)criminals_by_dist [i] + = cities [l-1];'是错的。如果'l == 0'呢?那么“l-1”是什么?稍后在同一个循环中与'r'相同。 –
'if(l> = 0)criminals_by_dist [i] + = cities [l-1];'可能导致出界限访问。 –