Java for LeetCode 028 Implement strStr()

Implement strStr().

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

解题思路一：

暴力枚举，JAVA实现如下：

static public int strStr(String haystack, String needle) {
for(int i=0;i<=haystack.length()-needle.length();i++)
if(haystack.substring(i, i+needle.length()).equals(needle))
return i;
return -1;
}

解题思路二：

经典的KMP算法

参考链接：
http://kb.cnblogs.com/page/176818/ http://blog.csdn.net/zxasqwedc/article/details/41956381
JAVA 实现

static public int strStr(String haystack, String needle) {
int[] next = new int[needle.length()+1];
int i = 0, j = -1;
//i可以代表索引，其实也可以理解为一个间隔，j代表前面已匹配的
//next[1,next.length-1] 恰好表示部分匹配值
next[0] = -1;
while (i < needle.length()) {
if (j == -1 || needle.charAt(i) == needle.charAt(j)) {
i++;
j++;
next[i] = j;
}
else
j = next[j];
}
i = 0;
j = 0;
while (i < haystack.length()) {
if (j == -1 || haystack.charAt(i) == needle.charAt(j)) {
i++;
j++;
} else
j = next[j];//是next[j]不是next[j-1]，算法核心
if (j == needle.length())
return i - needle.length();
}
return -1;
}