## 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(final String haystack, final String needle) {
if (needle.isEmpty()) return 0;

final int N = haystack.length() - needle.length() + 1;
for (int i = 0; i < N; i++) {
int j = i;
int k = 0;
while (j < haystack.length() && k < needle.length() &&
haystack.charAt(j) == needle.charAt(k)) {
j++;
k++;
}
if (k == needle.length()) return i;
}
return -1;
}
}

### KMP

// Implement strStr()
// KMP，时间复杂度O(N+M)，空间复杂度O(M)
public class Solution {
public int strStr(final String haystack, final String needle) {
return kmp(haystack, needle);
}

/*
* 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
*/
private static void compute_prefix(final String pattern, final int[] next) {
int i;
int j = -1;

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

if (pattern.charAt(i) == pattern.charAt(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
*/
private static int kmp(final String text, final String pattern) {
int i;
int j = -1;
final int n = text.length();
final int m = pattern.length();
if (n == 0 && m == 0) return 0; /* "","" */
if (m == 0) return 0;  /* "a","" */
int[] next = new int[m];

compute_prefix(pattern, next);

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

if (text.charAt(i) == pattern.charAt(j + 1)) j++;
if (j == m - 1) {
return i-j;
}
}

return -1;
}
}