理解计算最长围栏“|-----|-|”的函数的问题

Question

我有一个活动：

让我们考虑以下测试用例：

|------|-|
      ^^^^ 4

我认为它应该返回 4，但实际上它返回 2。

不正确的尝试：（底部调用者）

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#define MAX 10

//Prototypes: 
int longestFence(char input[], int size);

int main (void)
{
    char fenceString[] = "|------|-|"; // size is 10
    
    printf("%s → %d", fenceString, longestFence(fenceString, 10));
}

int longestFence (char input[], int size)
{
    int longestFence = 1; //Assume the lowest length (similar to Max activity)
    int count = 1;
    
    for (int i = 1; i < size;) //Start comparing from second index
    {
        if (input[i] + input[i - 1] == 169)  //Compare each index with the previous index
        {
            count++;
            i++;
        }
        
        else 
        {
            if (count > longestFence)
                longestFence = count;
            
            count = 1; //reset counting
            i++;
        }
    }

    return longestFence; // It should return 4 not 2
}

我修改了该函数，将 else 内部的条件也添加到 else 的外部，现在它按预期工作并返回 4，但是如何呢？我无法理解这种修改背后的逻辑...

工作尝试：

int longestFence (char input[], int size)
{
    int longestFence = 1; //Assume the lowest length (similar to Max activity)
    int count = 1;
    
    for (int i = 1; i < size;) //Start comparing from second index
    {
        if (input[i] + input[i - 1] == 169)  //Compare each index with the previous index
        {
            count++;
            i++;
        }
        
        else 
        {
            if (count > longestFence) //repeat this condition outside of else
                longestFence = count;
            
            count = 1; //reset counting
            i++;
        }

        //FIX :
        //The condition is repeated, now it's solved!
        if (count > longestFence)
            longestFence = count;
    }

    return longestFence; // It returns 4 as it should.
}

Answer 1

要调试此问题，您应该在纸上或其他地方绘制出变量随时间变化的状态。

您会发现您从未设置

longestFence

if 对于在字符串末尾找到的栅栏。

第二个片段起作用的原因不是你检查了两次；而是你检查了两次。而是您在不同的位置进行了检查。

实际的修复是这样的：

if (input[i] + input[i - 1] == 169)
{
    count++;
    // HERE
    if (count > longestFence) 
        longestFence = count;
}
else 
{
    count = 1; //reset counting
    // NOT HERE.
}

// COULD BE HERE INSTEAD, BUT NOT BEST

---

您的代码有第二个错误：它返回

1

表示空字符串。

---

我会怎么写：

size_t longestFence( const char *s ) {
   size_t max = 0;
   size_t len = 0;
   char prev = 0;
   for ( ; *s; ++s ) {
      if ( *s != prev ) {
         prev = *s;
         ++len;
         if ( max < len )
            max = len;
      } else {
         len = 1;
      }
   }

   return max;
}