## Implement strStr()

### 描述

Implement strStr().

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

### 暴力匹配

// Implement strStr()
// 暴力解法，时间复杂度O(N*M)，空间复杂度O(1)
class Solution {
public:
int strStr(const string& haystack, const string& needle) {
if (needle.empty()) return 0;

const int N = haystack.size() - needle.size() + 1;
for (int i = 0; i < N; i++) {
int j = i;
int k = 0;
while (j < haystack.size() && k < needle.size() && haystack[j] == needle[k]) {
j++;
k++;
}
if (k == needle.size()) return i;
}
return -1;
}
};

### KMP

// Implement strStr()
// KMP，时间复杂度O(N+M)，空间复杂度O(M)
class Solution {
public:
int strStr(const string& haystack, const string& needle) {
return kmp(haystack.c_str(), needle.c_str());
}
private:
/*
* @brief 计算部分匹配表，即next数组.
*
* @param[in] pattern 模式串
* @param[out] next next数组
* @return 无
*/
static void compute_prefix(const char *pattern, int next[]) {
int i;
int j = -1;
const int m = strlen(pattern);

next[0] = j;
for (i = 1; i < m; i++) {
while (j > -1 && pattern[j + 1] != pattern[i]) j = next[j];

if (pattern[i] == pattern[j + 1]) j++;
next[i] = j;
}
}

/*
* @brief KMP算法.
*
* @param[in] text 文本
* @param[in] pattern 模式串
* @return 成功则返回第一次匹配的位置，失败则返回-1
*/
static int kmp(const char *text, const char *pattern) {
int i;
int j = -1;
const int n = strlen(text);
const int m = strlen(pattern);
if (n == 0 && m == 0) return 0; /* "","" */
if (m == 0) return 0;  /* "a","" */
int *next = (int*)malloc(sizeof(int) * m);

compute_prefix(pattern, next);

for (i = 0; i < n; i++) {
while (j > -1 && pattern[j + 1] != text[i]) j = next[j];

if (text[i] == pattern[j + 1]) j++;
if (j == m - 1) {
free(next);
return i-j;
}
}

free(next);
return -1;
}
};