Implement strStr().

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

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解题思路：

解题想法是，从haystack的第一个位置，开始逐个判断是不是子串。如果整个子串都匹配了，那么就返回，否则继续往下挪位置。

Complexity: O(mn) , m is the length of haystack, n is the length of needle

还有一个KMP解法，算法复杂度很好，但目前没研究。以后再说。

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Java code:

public int strStr(String haystack, String needle) {        if( haystack == null || needle == null || needle.length() == 0 ) {             return 0;        }        for(int i = 0; i < haystack.length(); i++) {            if(i + needle.length() > haystack.length()) {                return -1;            }            int m = i;            for(int j = 0; j < needle.length(); j++) {                if(needle.charAt(j) == haystack.charAt(m)){                    if(j == needle.length()-1) {                        return i;                    }                    m++;                }else {                    break;                }            }        }        return -1;    }

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Reference:

1. http://www.programcreek.com/2012/12/leetcode-implement-strstr-java/

2. http://jakeboxer.com/blog/2009/12/13/the-knuth-morris-pratt-algorithm-in-my-own-words/

3. http://www.cnblogs.com/springfor/p/3896469.html