KMP

KMP算法记住一句话：构造最大长度后缀数组

1. 根据当前的next[j] = k; 进而推断next[j+1] = ?; 思想是：如果当前P(j) == P(k), 就把最长后缀伸长一点；否则区间变小。

2.KMP是和getNext(); similar

3.记住那句话：构造最大长度后缀

4.对getNext();优化

import java.io.*;

public class Test
{
	public static void main(String args[]) throws IOException
	{
		BufferedReader buf = new BufferedReader(new InputStreamReader(System.in));
		
		String str1 = buf.readLine();
		String str2 = buf.readLine();
		
		KMP(str1, str2);
	}
	
// getNext();	
	public static int[] getNext(char x[])
	{
		int next[] = new int[x.length];
		next[0] = -1;
		
		int i = 0;
		int j = -1;
		while (i < next.length - 1) // take care
		{
			if (j == -1 || x[i] == x[j])
			{
				++i;
				++j; // ++i, ++j, 表明最长后缀伸长
				next[i] = j;
			}
			else
			{
				j = next[j];
			}
		}
		return next;
		
	}
	
// KMP
	public static void KMP(String t, String p)
	{
		char ch[] = p.toCharArray();
		int next[] = getNext(ch);
		
		int i = 0;
		int j = 0;
		while (i < t.length() && j <p.length())
		{
			if (j == -1 || t.charAt(i) == p.charAt(j))
			{
				++i;
				++j;
			}
			else
			{
				j = next[j];
			}
		}
		if (j == p.length())
			System.out.println(i - p.length());
		else
			System.out.println("no");
	}
}

修正next【】的结果：

public static int[] getNext(char x[])
	{
		int next[] = new int[x.length];
		next[0] = -1;
		
		int i = 0;
		int j = -1;
		while (i < next.length - 1) // take care
		{
			if (j == -1 || x[i] == x[j])
			{
				++i;
				++j;
				if (x[i] == x[j])
					next[i] = next[j];
				else
					next[i] = j;
					
			}
			else
			{
				j = next[j];
			}
		}
		return next;
		
	}