java版LCS算法

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.HashSet;
import java.util.List;

/**
 * LCS(Longest Common Subsequence)求最長公共字符串系列
 * 
 * @author zzf
 * 
 */
public class LCS {
	private final static String[] empty = new String[0];

	/**
	 * @return 最長公共字符串(可能有多個長度相同的)
	 */
	public static String[] lcs(String s1, String s2, boolean noRepeat) {
		//排除常用情況
		if (s1 == null || s1.length() == 0 || s2 == null || s2.length() == 0) {
			return empty;
		}
		int len1 = s1.length();
		int len2 = s2.length();
		if (len1==len2&&s1.equals(s2)) {
			return new String[] { s1 };
		}
		if (len1 > len2 && s1.indexOf(s2) != -1) {
			return new String[] { s2 };
		}
		if (len2 > len1 && s2.indexOf(s1) != -1) {
			return new String[] { s1 };
		}
		//構造對比矩陣
		char[] chs1 = s1.toCharArray();
		char[] chs2 = s2.toCharArray();
		boolean[][] as = new boolean[len1][len2];
		for (int i = 0; i < len1; i++) {
			char ch1 = chs1[i];
			for (int j = 0; j < len2; j++) {
				char ch2 = chs2[j];
				as[i][j] = ch1 == ch2;
			}
		}
		// 收集正向\對角線
		int max = 0;
		Collection<String> buf = noRepeat ? (new HashSet<String>())
				: (new ArrayList<String>());
		for (int i = 0; i < len1; i++) {
			for (int j = 0; j < len2; j++) {
				int len = lcsLength(i, j, len1, len2, as);
				if (len > 0 && len >= max) {
					max = len;
					buf.add(s1.substring(i, i + len));
				}
			}
		}
		// 取最長對角線(最長匹配字符)
		List<String> ret = new ArrayList<String>();
		for (String s : buf) {
			if (s.length() == max) {
				ret.add(s);
			}
		}
		return ret.toArray(empty);
	}

	/**
	 * 遞歸計算長度(對角線)
	 */
	private static int lcsLength(int i, int j, int len1, int len2,
			boolean[][] as) {
		if (as[i][j]) {
			int c = 1;
			i++;
			j++;
			if (i < len1 && j < len2) {
				c += lcsLength(i, j, len1, len2, as);
			}
			return c;
		}
		return 0;
	}

	public static void main(String[] args) {
		{
			String s1 = "12345";
			String s2 = "12456";
			// 輸出[45,12]
			System.out.println(Arrays.asList(lcs(s1, s2, true)));
		}
		{
			String s1 = "123456";
			String s2 = "1205678";
			// 輸出[56,12]
			System.out.println(Arrays.asList(lcs(s1, s2, true)));
		}
	}
}