LeetCode题解：Implement strStr()

Implement strStr()

Implement strStr().

Returns a pointer to the first occurrence of needle in haystack, or null if needle is not part of haystack.

思路：

首先想到的是KMP算法。不过似乎不是很成功，提交了之后总是报Runtime error，但是我这里无论本地机器跑还是在valgrind上跑都不报错。奇怪的问题。

题解：

const char* strStr (const char* S, const char* W)
{
int len_W;
int* T; // table
int i, j, m;

if (S == NULL || W == NULL)
return NULL;

len_W = strlen (W);

if (len_W == 0)
return 0;
else if (len_W == 1)
{
while (*S != 0 && *S != *W)
++S;
return *S == 0 ? NULL : S;
}

// build the table
T = (int*) malloc (len_W * sizeof (int));
T[0] = -1;  // must forward!!
T[1] = 0;

i = 2;
j = 0;
while (i < len_W)
if (W[i - 1] == W[j])
++j, T[i] = j, ++i;
else if (j > 0)
j = T[j];
else
++j, ++i;

// KMP
m = 0, i = 0;
while (S[m + i] != 0)
{
if (W[i] == S[m + i])
{
if (i == len_W - 1)
break;
++i;
}
else
// fall back
m = m + i - T[i], i = T[i] > -1 ? T[i] : 0;
}

free (T);

if (S[m + i] == 0)
return NULL;
else
return S + m;
}

思路：

重新测试一下，用最简单的暴力法。结果超时了。

题解：

class Solution {
public:
char* strStr (char* s, char* w)
{
if (s == nullptr || w == nullptr)
return nullptr;

while (*s != 0)
{
while (*s != *w && *s != 0) ++s;

char* sforward = s;
char* wforward = w;
while (*sforward == *wforward &&
*sforward != 0 && *wforward != 0)
++sforward, ++wforward;

if (*wforward == 0)
return s;
else
++s;
}
return nullptr;
}
};

思路：

LeetCode有时候需要我们耍一下流氓。在检查的时候先看一下最后一个字符是不是match，如果不match就跳过。这其实没什么大意思。

题解：

class Solution {
public:
char* strStr (char* s, char* w)
{
if (s == nullptr || w == nullptr)
return nullptr;

if (*w == 0)
return s;

const int LEN_S = std::strlen (s);
const int LEN_W = std::strlen (w);

char* s_iter = s;
char* s_iter_end = s + (LEN_S - LEN_W);
while (s_iter <= s_iter_end)
{
while (*s_iter != *w && s_iter < s_iter_end) ++s_iter;

char* sforward = s_iter;
char* wforward = w;

if (* (s_iter + LEN_W - 1) == * (w + LEN_W - 1))
{
while (*sforward == *wforward && *wforward != 0)
++sforward, ++wforward;

if (*wforward == 0)
return s_iter;
}

++s_iter;
}
return nullptr;
}
};