字符串匹配总结

转载请注明作者与出处： franciscolv http://www.cnblogs.com/franciscolv/archive/2011/11/20/2255817.html。   

字符串匹配总结了朴素字符串匹配算法和KMP匹配算法。朴素字符串匹配算法主要问题在于回溯。KMP优化了最糟糕情况下朴素算法的回溯代价。

package SortSet;

/**
 * 匹配字符串
 * 
 * @author franciscolv
 * 
 */
public class StringMatch {

    public static void main(String[] args) {
        int result =findPos("helloworld".toCharArray(),"llo".toCharArray());
        int result2 = findPosKMP("kabdabcm".toCharArray(), "abdabc".toCharArray());
        if (result == -1||result2 == -1)
            System.out.println("has no such string");

    }

    /**
     * o(m+n) 极限情况O(mn)
     * 
     * @param s
     * @param target
     * @return
     */
    public static int findPos(char[]c, char[]t) {
        int i = 0, j = 0;
        while (i < c.length && j < t.length) {
            if (c[i] == t[j]) {
                i++;
                j++;
            } else {
                i = i - j + 1;
                j = 0;
            }
        }
        if (j == t.length) {
            for (int k = i - j; k < i; k++)
                System.out.print(c[k]);
            System.out.println();
            return i - j;
        }
        return -1;
    }

    /**
     * o(m+n)
     * 
     * @param s
     * @param t0
     * @return
     */
    public static int findPosKMP(char[]c, char[]t) {    
        int[] next=  initNext(t);
        int i = 0, j = 0;
        while (i < c.length && j < t.length) {
            if (j == -1 || c[i] == t[j]) {
                j++;
                i++;
            } else
                j = next[j];
        }
        if (j == t.length) {
            for (int k = i - j; k < i; k++)
                System.out.print(c[k]);
            System.out.println();
            System.out.println(i - j);
            return i - j;
        }
        return -1;
    }

    private static int[] initNext(char[] t) {
        int[] next = new int[t.length-1];
        int j = 0, k = -1;
        next[0] = -1;
        while (j < t.length - 1) {
            if (k == -1 || t[j] == t[k]) {
                j++;
                k++;
                next[j] = k;
            } else
                k = next[k];
        }
        for (int i : next)
            System.out.print(i + " ");
        System.out.println();
        return next;
    }
}