Implement strStr

問題描述

Implement strStr().

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

思考：字符串匹配，KMP算法

想法

• 1、暴力匹配算法效率肯定不高
• 2、重溫一下KMP算法，next數組 http://blog.csdn.net/v_july_v/article/details/7041827
• 3、學習一下BM算法和sunday算法（一般效率高於KMP）

代碼

public class Solution {
    public int strStr(String haystack, String needle) {
        int hsl = haystack.length();
        int ndl = needle.length();
        if(ndl < 1)
            return 0;
        int[] next =  getNextval(needle);
        int i = 0, j = 0;
        while(i < hsl && j < ndl){ 
            if(j == -1 || haystack.charAt(i) == needle.charAt(j)){
                i++;
                j++; 
            }
            else
                j = next[j];
        }
        return j == ndl? i - j: -1;
    }

    private int[] getNextval(String p){
        int len = p.length();
        int j = 0;
        int[] next = new int[len];
        int k = next[0] = -1;     
        while( j < len - 1){          
            if(k == - 1 || p.charAt(j) == p.charAt(k)){
                j++;
                k++;
                if(p.charAt(j) != p.charAt(k))
                    next[j] = k;
                else
                    next[j] = next[k];
            }
            else
                k = next[k];
        }
        return next;
    }
}