LeetCode: strStr

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

我以为不就是找个字符串吗，扫描一遍不就行了。。后来才发现，原来面试的时候不让用这种方法，都考出花了。

所以学习了一下KMP算法。具体内容有链接。

public static String kmp(String haystack, String needle) {
        if (needle.length() == 0) return haystack;
        if (haystack.length() == 0) return null;
        if (haystack.length() < needle.length()) return null;
        
        int[] next = new int[needle.length()];
        next = kmpNext(needle);
        
        for (int i = 0, j = 0; i < haystack.length(); i++) {
          while (j > 0 && haystack.charAt(i) != needle.charAt(j)) {
              j = next[j - 1];
          }
          
          if (haystack.charAt(i) == needle.charAt(j))
              j++;
          //match successfully
          if (j == needle.length())
              return haystack.substring(i-j+1);
      }
      return null;
    }

    public static int[] kmpNext(String needle) {
        int[] next = new int[needle.length()];
        if (needle.length() == 0) return next;
        next[0] = 0;
        for (int i = 1, j = 0; i < needle.length(); ++i) {
            while (j > 0 && needle.charAt(i) != needle.charAt(j)) {
                j = next[j - 1];
            }

            if (needle.charAt(i) == needle.charAt(j)) 
                ++j;

            next[i] = j;
        }
        return next;
    }

 

http://biaobiaoqi.me/blog/2013/05/25/kmp-algorithm/

http://blog.csdn.net/v_july_v/article/details/7041827