KMP(Knuth–Morris–Pratt) Algorithm

文章 String S, 要查找的字符串 String W

重点：Get int T[] from W

用变量 c 来表示当前比较到的 W 中前面的字符,
i 表示当前要处理的 T 中的 index,
int[] T = new int[W.length];
T[0] = -1; T[1] = 0;
c=0; i=2

example:
array:    a    b    a    b    c    d    a    b    a    b    a    b
idx    :    0    1    2    3    4    5    6    7    8    9   10  11

case 1:  W[c] == W[i-1] --> T[i] = ++c; i++;
case 2:  W[c] != W[i-1] && c>0 --> c = T[c];
case 3:  W[c] != W[i-1] && c==0 --> T[i] = 0; i++;

过程
array: a    b    a    b    c    d    a    b    a    b    a    b
idx   : 0    1    2    3    4    5    6    7    8    9   10  11

i:	2	3	4	5	5	6	7	8	9	10	11	11
case:	3	1	1	2	3	3	1	1	1	1	2	1
pre-c:	0	0	1	2	0	0	0	1	2	3	4	2
after-c:	0	1	2	0	0	0	1	2	3	4	2	3
比较:	0 & 0	0 & 2	1 & 3	2 & 4	0 & 4	0 & 5	0 & 6	1 & 7	2 & 8	3 & 9	4 & 10	2 & 10

Related Problems: Implement strStr(), Shortest Palindrome

最后 po 一段自己写的 leetcode 上 implement strStr() 的代码

public class Solution {
    public int strStr(String haystack, String needle) {
        if (needle == null || needle.length() == 0) return 0;
        
        int ndlen = needle.length();
        int hslen = haystack.length();
        
        if (ndlen == 1) {
            // only need to find the character in heystack
            return strChar(haystack, needle.charAt(0));
        }
        
        int[] table = buildKMPTable(needle);
        
        // KMP algorithm based on helper table
        int ndidx = 0, hsidx = 0;
        while (hsidx < hslen) {
            if (haystack.charAt(hsidx) == needle.charAt(ndidx)) {
                hsidx++;
                ndidx++;
                if (ndidx == ndlen) return hsidx - ndlen;
            } else if (table[ndidx] != -1) {
                ndidx = table[ndidx];
            } else {
                hsidx++;
            }
        }
        return -1;
    }
    
    private int strChar(String str, char c) {
        if (str == null || str.length() == 0) return -1;
        for (int i = 0; i < str.length(); i++) {
            if (str.charAt(i) == c) return i;
        }
        return -1;
    }
    
    /* KMP table[i] contain the idx position that 
     * when str[j] != word[i], compare str[j] & word[table[i]]
     */
    private int[] buildKMPTable(String needle) {
        int[] table = new int[needle.length()];
        
        // needle.length() > 1
        table[0] = -1;
        table[1] = 0;
        int idx = 0;
        int i = 2;
        while(i < needle.length())  {
            char pre = needle.charAt(idx);
            char cur = needle.charAt(i - 1);
            if (pre == cur) { // case 1. word[idx] == word[i-1] 
                table[i++] = ++idx;
            } else {
                if (idx == 0) { // case 2. word[idx] != word[i-1] && idx == 0
                    table[i++] = 0;
                } else { // case 3. word[idx] != word[i-1] && idx > 0
                    idx = table[idx];
                }
            }
        }
        return table;
    }
}