[Algorithms] KMP

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 inable 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.

You may use this code to solve the problem Implement strStr() on LeetCode. My accepted 4ms C++ code using KMP is as follows.

class Solution {
public:
    int strStr(string haystack, string needle) {
        int m = needle.length(), n = haystack.length();
        if (!m) return 0;
        vector<int> lps = kmpProcess(needle);
        for (int i = 0, j = 0; i < n; ) {
            if (needle[j] == haystack[i]) {
                i++;
                j++;
            }
            if (j == m) return i - j;
            if (i < n && needle[j] != haystack[i]) {
                if (j) j = lps[j - 1];
                else i++;
            }
        }
        return -1;
    }
private:
    vector<int> kmpProcess(string needle) {
        int m = needle.length();
        vector<int> lps(m, 0);
        for (int i = 1, len = 0; i < m; ) {
            if (needle[i] == needle[len])
                lps[i++] = ++len;
            else if (len) len = lps[len - 1];
            else lps[i++] = 0;
        }
        return lps;
    }
};