• [LeetCode] Implement strStr()


    Well, the problem does not aim for an advanced algorithm like KMP but only a clean brute-force algorithm. So we can traverse all the possible starting points of haystack (from 0 tohaystack.length() - needle.length()) and see if the following characters in haystack match those of needle.

    The code is as follows.

     1 class Solution {
     2 public: 
     3     int strStr(string haystack, string needle) {
     4         int m = haystack.length(), n = needle.length();
     5         if (!n) return 0;
     6         for (int i = 0; i < m - n + 1; i++) {
     7             int j = 0;
     8             for (; j < n; j++)
     9                 if (haystack[i + j] != needle[j])
    10                     break;
    11             if (j == n) return i;
    12         }
    13         return -1;
    14     }
    15 };

    Of course, you may challenge yourself implementing the KMP algorithm for this problem.

    KMP is a classic and yet notoriously hard-to-understand algorithm. However, I think the following two links give nice explanations. You may refer to them.

    1. KMP on jBoxer's blog;
    2. KMP on geeksforgeeks, with a well-commented C code.

    I am sorry that I am still unable to give a personal explanation of the algorithm. I only read it from the two links above and mimic the code in the second link.

    My accepted C++ code using KMP is as follows. Well, it also takes 4ms -_-

     1 class Solution {
     2 public:
     3     int strStr(string haystack, string needle) {
     4         int m = haystack.length(), n = needle.length();
     5         if (!n) return 0;
     6         vector<int> lps = kmpProcess(needle);
     7         for (int i = 0, j = 0; i < m; ) {
     8             if (haystack[i] == needle[j]) { 
     9                 i++;
    10                 j++;
    11             }
    12             if (j == n) return i - j;
    13             if (i < m && haystack[i] != needle[j]) {
    14                 if (j) j = lps[j - 1];
    15                 else i++;
    16             }
    17         }
    18         return -1;
    19     }
    20 private:
    21     vector<int> kmpProcess(string& needle) {
    22         int n = needle.length();
    23         vector<int> lps(n, 0);
    24         for (int i = 1, len = 0; i < n; ) {
    25             if (needle[i] == needle[len])
    26                 lps[i++] = ++len;
    27             else if (len) len = lps[len - 1];
    28             else lps[i++] = 0;
    29         }
    30         return lps;
    31     }
    32 };
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  • 原文地址:https://www.cnblogs.com/jcliBlogger/p/4557435.html
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