[LintCode] #79 Longest Common Substring

public class Solution {
    /**
     * @param A, B: Two string.
     * @return: the length of the longest common substring.
     */
     
    // state: f[i][j] - length of LCS of A.substring(0, i] and B.substring(0, j]
    // function: f[i][j] = f[i - 1][j - 1] + 1, if A[i] == B[j]
    //                   = 0, if A[i] != B[j]
    // initialize: 
    // result: f[0...A.length - 1][0...B.length - 1]
    
    public int longestCommonSubstring(String A, String B) {
        if (A == null || B == null || A.length() == 0 || B.length() == 0) {
            return 0;
        }
        int max = 0;
        int[][] f = new int[A.length()][B.length()];
        for (int i = 0; i < A.length(); i++) {
            f[i][0] = A.charAt(i) == B.charAt(0) ? 1 : 0;
            max = Math.max(max, f[i][0]);
        }
        for (int j = 0; j < B.length(); j++) {
            f[0][j] = A.charAt(0) == B.charAt(j) ? 1 : 0;
            max = Math.max(max, f[0][j]);
        }
        for (int i = 1; i < A.length(); i++) {
            for (int j = 1; j < B.length(); j++) {
                if (A.charAt(i) == B.charAt(j)) {
                    f[i][j] = f[i - 1][j - 1] + 1;
                } else {
                    f[i][j] = 0;
                }
                max = Math.max(max, f[i][j]);
            }
        }
        return max;
    }
}