A-M baby

Task/Longest-common-subsequence/Java/longest-common-subsequence-1.java

@@ -0,0 +1,14 @@
+public static String lcs(String a, String b){
+    int aLen = a.length();
+    int bLen = b.length();
+    if(aLen == 0 || bLen == 0){
+        return "";
+    }else if(a.charAt(aLen-1) == b.charAt(bLen-1)){
+        return lcs(a.substring(0,aLen-1),b.substring(0,bLen-1))
+            + a.charAt(aLen-1);
+    }else{
+        String x = lcs(a, b.substring(0,bLen-1));
+        String y = lcs(a.substring(0,aLen-1), b);
+        return (x.length() > y.length()) ? x : y;
+    }
+}

Task/Longest-common-subsequence/Java/longest-common-subsequence-2.java

@@ -0,0 +1,31 @@
+public static String lcs(String a, String b) {
+    int[][] lengths = new int[a.length()+1][b.length()+1];
+
+    // row 0 and column 0 are initialized to 0 already
+
+    for (int i = 0; i < a.length(); i++)
+        for (int j = 0; j < b.length(); j++)
+            if (a.charAt(i) == b.charAt(j))
+                lengths[i+1][j+1] = lengths[i][j] + 1;
+            else
+                lengths[i+1][j+1] =
+                    Math.max(lengths[i+1][j], lengths[i][j+1]);
+
+    // read the substring out from the matrix
+    StringBuffer sb = new StringBuffer();
+    for (int x = a.length(), y = b.length();
+         x != 0 && y != 0; ) {
+        if (lengths[x][y] == lengths[x-1][y])
+            x--;
+        else if (lengths[x][y] == lengths[x][y-1])
+            y--;
+        else {
+            assert a.charAt(x-1) == b.charAt(y-1);
+            sb.append(a.charAt(x-1));
+            x--;
+            y--;
+        }
+    }
+
+    return sb.reverse().toString();
+}