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.

分析

暴力算法的复杂度是 O(m*n)，代码如下。更高效的的算法有KMP算法、Boyer-Mooer算法和Rabin-Karp算法。面试中暴力算法足够了，一定要写得没有BUG。

暴力匹配

  1. // Implement strStr()
  2. // 暴力解法，时间复杂度O(N*M)，空间复杂度O(1)
  3. class Solution {
  4.     public int strStr(final String haystack, final String needle) {
  5.         if (needle.isEmpty()) return 0;
  7.         final int N = haystack.length() - needle.length() + 1;
  8.         for (int i = 0; i < N; i++) {
  9.             int j = i;
  10.             int k = 0;
  11.             while (j < haystack.length() && k < needle.length() && 
  12.                     haystack.charAt(j) == needle.charAt(k)) {
  13.                 j++;
  14.                 k++;
  15.             }
  16.             if (k == needle.length()) return i;
  17.         }
  18.         return -1;
  19.     }
  20. }

KMP

  1. // Implement strStr()
  2. // KMP，时间复杂度O(N+M)，空间复杂度O(M)
  3. public class Solution {
  4.     public int strStr(final String haystack, final String needle) {
  5.         return kmp(haystack, needle);
  6.     }
  8.     /*
  9.      * 计算部分匹配表，即next数组.
  10.      *
  11.      * @param[in] pattern 模式串
  12.      * @param[out] next next数组
  13.      * @return 无
  14.      */
  15.     private static void compute_prefix(final String pattern, final int[] next) {
  16.         int i;
  17.         int j = -1;
  19.         next[0] = j;
  20.         for (i = 1; i < pattern.length(); i++) {
  21.             while (j > -1 && pattern.charAt(j + 1) != pattern.charAt(i)) j = next[j];
  23.             if (pattern.charAt(i) == pattern.charAt(j + 1)) j++;
  24.             next[i] = j;
  25.         }
  26.     }
  28.     /*
  29.      * @brief KMP算法.
  30.      *
  31.      * @param[in] text 文本
  32.      * @param[in] pattern 模式串
  33.      * @return 成功则返回第一次匹配的位置，失败则返回-1
  34.      */
  35.     private static int kmp(final String text, final String pattern) {
  36.         int i;
  37.         int j = -1;
  38.         final int n = text.length();
  39.         final int m = pattern.length();
  40.         if (n == 0 && m == 0) return 0; /* "","" */
  41.         if (m == 0) return 0;  /* "a","" */
  42.         int[] next = new int[m];
  44.         compute_prefix(pattern, next);
  46.         for (i = 0; i < n; i++) {
  47.             while (j > -1 && pattern.charAt(j + 1) != text.charAt(i)) j = next[j];
  49.             if (text.charAt(i) == pattern.charAt(j + 1)) j++;
  50.             if (j == m - 1) {
  51.                 return i-j;
  52.             }
  53.         }
  55.         return -1;
  56.     }
  57. }

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原文: https://soulmachine.gitbooks.io/algorithm-essentials/content/java/string/strstr.html