Solved problems

Author: gouthampradhan

problems/src/dynamic_programming/CountDifferentPalindromicSubsequences.java

@@ -0,0 +1,61 @@
+package dynamic_programming;
+
+import java.util.Arrays;
+
+/** Created by gouthamvidyapradhan on 10/04/2021 */
+public class CountDifferentPalindromicSubsequences {
+  public static void main(String[] args) {
+    System.out.println(
+        new CountDifferentPalindromicSubsequences()
+            .countPalindromicSubsequences(
+                "abcdabcdabcdabcdabcdabcdabcdabcddcbadcbadcbadcbadcbadcbadcbadcba"));
+  }
+
+  private long[][][] DP;
+  final char[] chars = {'a', 'b', 'c', 'd'};
+  final int MOD = (int) 1e9 + 7;
+
+  public int countPalindromicSubsequences(String S) {
+    DP = new long[S.length()][S.length()][4];
+    for (int i = 0; i < S.length(); i++) {
+      for (int j = 0; j < S.length(); j++) {
+        Arrays.fill(DP[i][j], -1);
+      }
+    }
+    long result = 0L;
+    for (char c : chars) {
+      long r = dp(0, S.length() - 1, S, c);
+      result = ((result + r) % MOD);
+    }
+    return (int) result;
+  }
+
+  private long dp(int i, int j, String s, char c) {
+    if (i > j) return 0;
+    else if (DP[i][j][c - 'a'] != -1) return DP[i][j][c - 'a'];
+    else if (s.charAt(i) == s.charAt(j) && s.charAt(i) == c) {
+      if (i == j) return 1;
+      else {
+        long sum = 0L;
+        for (char aChar : chars) {
+          long r = dp(i + 1, j - 1, s, aChar);
+          if (aChar == c) {
+            r = ((r + 2) % MOD);
+          }
+          sum = ((sum + r) % MOD);
+        }
+        DP[i][j][c - 'a'] = sum;
+        return DP[i][j][c - 'a'];
+      }
+    } else if (s.charAt(i) == c) {
+      DP[i][j][c - 'a'] = dp(i, j - 1, s, c);
+      return DP[i][j][c - 'a'];
+    } else if (s.charAt(j) == c) {
+      DP[i][j][c - 'a'] = dp(i + 1, j, s, c);
+      return DP[i][j][c - 'a'];
+    } else {
+      DP[i][j][c - 'a'] = dp(i + 1, j - 1, s, c);
+      return DP[i][j][c - 'a'];
+    }
+  }
+}