字符串匹配KMP算法

KMP算法

KMP算法主要维护了一个最长前后缀的表，当待匹配串和匹配串的指针一起向前退的时候，遇到不相等的字符，待匹配串不用再回到起始位置的下一个位置重新匹配（暴力匹配），只需要查表更改匹配串的匹配指针继续向下走即可，从而将n*m的复杂度降低为接近n的复杂度。

实际代码

interface IMatch{
    int find();
}
class BaoLi implements IMatch{
    private String text;
    private String pattern;

    public BaoLi(String text, String pattern) {
        this.text = text;
        this.pattern = pattern;
    }

    @Override
    public int find() {
        int m = text.length();
        int n = pattern.length();
        int i = 0;
        while (i < m){
            int start = i;
            int j = 0;
            while(text.charAt(i) == pattern.charAt(j)){
                if(j == n-1)
                    return start;
                i++;
                j++;
            }
            i = ++start;
        }
        return -1;
    }
}

public class KMP implements IMatch{
    private String text;
    private String pattern;
    private int[] prefixTable;

    public KMP(String text, String pattern) {
        this.text = text;
        this.pattern = pattern;
        prefixTable = new int[pattern.length()];
        createPrefixTable();
        movePrefixTable();
    }

    @Override
    public int find() {
        int i = 0; // 这是指着text串的指针
        int j = 0; // 这是指着pattern串的指针
        int m = text.length();
        int n = pattern.length();
        while (i < m) {
            if (j == n - 1 && text.charAt(i) == pattern.charAt(j))
                return i - n + 1;
            if (text.charAt(i) == pattern.charAt(j)) {
                i++;
                j++;
            } else {
                j = prefixTable[j];
                if (j == -1) {
                    i++;
                    j++;
                }
            }
        }
        return -1;
    }

    private void createPrefixTable() {
        prefixTable[0] = 0;
        int len = 0;
        int i = 1;
        while (i < pattern.length()) {
            if (pattern.charAt(i) == pattern.charAt(len)) {
                prefixTable[i++] = ++len;
            } else {
                if (len > 0) {
                    // 关键
                    len = prefixTable[len - 1];
                } else {
                    prefixTable[i] = len;
                    i++;
                }
            }
        }
    }

    private void movePrefixTable() {
        int i;
        for (i = prefixTable.length - 1; i > 0; i--) {
            prefixTable[i] = prefixTable[i - 1];
        }
        prefixTable[0] = -1;
    }

    public static void main(String[] args) {
        String s1 = "aaaaaaaaasastiii";
        String s2 = "aaas";
        IMatch match1 = new KMP(s1, s2);
        IMatch match2 = new BaoLi(s1,s2);

        System.out.println(s1 + " " + s2 + " : " + match1.find());

        System.out.println(s1 + " " + s2 + " : " + match2.find());

    }
}