【LeetCode】028.Implement strStr()

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题目：

Implement strStr().

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

解答：

这是典型的字符串匹配问题，用KMP算法（不清楚的请自行百度），KMP算法的核心就是利用最长前缀串减少回溯次数。

OJ上这道题有个坑：patStr长度为0时，必须返回0，不论tarStr是否为空。

代码如下：

import java.util.Vector;public class Solution {    public int strStr(String haystack, String needle) {if(haystack == null || needle == null)return -1;if(needle.length() == 0)return 0;if(haystack.length() == 0)return -1;char[] tar = haystack.toCharArray();char[] pattern = needle.toCharArray();Vector<Integer> prefix = this.CptPrefix(pattern);int NOCM = 0; // Number of characters matchedfor (int i = 0; i < tar.length; i++) {while (NOCM > 0 && tar[i] != pattern[NOCM])NOCM = prefix.elementAt(NOCM);if (tar[i] == pattern[NOCM])NOCM++;if (NOCM == pattern.length) { // matchedreturn i - pattern.length + 1;}}return -1;}public Vector<Integer> CptPrefix(char[] c) {if (c == null || c.length == 0)return null;int NOCM = 0; // Number of Character Matchedint LOLP = 0; // Length of Longest PatterVector<Integer> prefix = new Vector<Integer>(c.length);prefix.add(0);prefix.add(0);for (NOCM = 2; NOCM < c.length; NOCM++) {while (LOLP > 0 && c[NOCM-1] != c[LOLP])LOLP = prefix.elementAt(LOLP);if (c[NOCM-1] == c[LOLP])LOLP++;prefix.add(NOCM, LOLP);}return prefix;}}

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