Java LCS LCSAlgorithm(String a, String b)

Description

LCS Algorithm

License

Open Source License

Declaration

public static String LCSAlgorithm(String a, String b) 

Method Source Code

//package com.java2s;
//License from project: Open Source License 

public class Main {
    private static final int NEITHER = 0;
    private static final int UP = 1;
    private static final int LEFT = 2;
    private static final int UP_AND_LEFT = 3;

    public static String LCSAlgorithm(String a, String b) {
        int n = a.length();
        int m = b.length();
        int S[][] = new int[n + 1][m + 1];
        int R[][] = new int[n + 1][m + 1];
        int ii, jj;

        // It is important to use <=, not <.  The next two for-loops are initialization
        for (ii = 0; ii <= n; ++ii) {
            S[ii][0] = 0;/*from w  w w . ja  v a2  s  .  co m*/
            R[ii][0] = UP;
        }
        for (jj = 0; jj <= m; ++jj) {
            S[0][jj] = 0;
            R[0][jj] = LEFT;
        }

        // This is the main dynamic programming loop that computes the score and
        // backtracking arrays.
        for (ii = 1; ii <= n; ++ii) {
            for (jj = 1; jj <= m; ++jj) {

                if (a.charAt(ii - 1) == b.charAt(jj - 1)) {
                    S[ii][jj] = S[ii - 1][jj - 1] + 1;
                    R[ii][jj] = UP_AND_LEFT;
                } else {
                    S[ii][jj] = S[ii - 1][jj - 1] + 0;
                    R[ii][jj] = NEITHER;
                }

                if (S[ii - 1][jj] >= S[ii][jj]) {
                    S[ii][jj] = S[ii - 1][jj];
                    R[ii][jj] = UP;
                }

                if (S[ii][jj - 1] >= S[ii][jj]) {
                    S[ii][jj] = S[ii][jj - 1];
                    R[ii][jj] = LEFT;
                }
            }
        }

        // The length of the longest substring is S[n][m]
        ii = n;
        jj = m;
        int pos = S[ii][jj] - 1;
        char lcs[] = new char[pos + 1];

        // Trace the backtracking matrix.
        while (ii > 0 || jj > 0) {
            if (R[ii][jj] == UP_AND_LEFT) {
                ii--;
                jj--;
                lcs[pos--] = a.charAt(ii);
            } else if (R[ii][jj] == UP) {
                ii--;
            } else if (R[ii][jj] == LEFT) {
                jj--;
            }
        }

        return new String(lcs);
    }
}

Related

1. lcs(String X, String Y, int m, int n)
2. lcs1(int[] A, int[] B)
3. lcs2(int[] A, int[] B, int m, int n)
4. lcs3(int[] A, int[] B)
5. lcs4(int[] A, int[] B)
6. lcse(String str1, String str2)
7. LCSIterative(Character[] X, int i, Character Y[], int j, int c[][])
8. LCSRecursive(Character[] X, int i, Character Y[], int j, int c[][])