KMP算法

KMP是字符子串的查找算法

代码：
 

public class KMP {

    /**
     * 暴力搜索
     * @param source  源串
     * @param pattern 匹配串
     * @return
     */
    public static int roughSearch(String source, String pattern){

        int m = source.length();
        int n = pattern.length();

        if(m < n)
            return -1;

        int j=0;
        for(int i=0; i<=m-n; i++){
            for(j=0;j<n;j++){
                if(pattern.charAt(j) != source.charAt(i+j)){
                    break;
                }


            }

            if(j == n)
                return i;

        }


        return -1;
    }

    /**
     * Compute lps array to maintain size of suffix which is same as prefix
     * get the array that stores the longest subarray whose prefix is also its suffix
     * @param pattern
     * @return
     */
    private int[] computeLpsArray(char pattern[]){
        int [] lps = new int[pattern.length];
        int index =0;
        for(int i=1; i < pattern.length;){
            if(pattern[i] == pattern[index]){
                lps[i] = index + 1;
                index++;
                i++;
            }else{
                if(index != 0){
                    index = lps[index-1];
                }else{
                    lps[i] =0;
                    i++;
                }
            }
        }
        return lps;
    }

    /**
     *
     * @param text
     * @param pattern
     * @return
     */
    public int doKMP(char []text, char []pattern){
        int lps[] = computeLpsArray(pattern);
        int i=0;
        int j=0;
        int m = text.length;
        int n = pattern.length;
        while(i < m && j < n){
            if(text[i] == pattern[j]){
                i++;
                j++;
            }else{
                if(j!=0){
                    j = lps[j-1];
                }else{
                    i++;
                }
            }
        }
        if(j == n){
            return i-n;
        }
        return -1;
    }

    public static void main(String[] args) {
        KMP kmp = new KMP();
        String S = "goodgoogle";
        String T = "google";

        int index = kmp.doKMP(S.toCharArray(), T.toCharArray());

        if(index > 0){
            System.out.printf("find substrring, index is %d.", index);
        }else{
            System.out.printf("Not found the substring!! code: %d", index);
        }

    }
}