feat: updated LCS problem

longest-common-subsequence/src/TLE.cpp

@@ -2,16 +2,23 @@
 
 using namespace std;
 
-int solve(string &s1, string &s2, int i = 0, int j = 0)
+pair<int, string> solve(string &s1, string &s2, int i = 0, int j = 0)
 {
     if (i == s1.size() || j == s2.size())
-        return 0;
+        return {0, ""};
 
     if (s1[i] == s2[j])
     {
-        return (solve(s1, s2, i + 1, j + 1) + 1);
+        auto [length, sub] = solve(s1, s2, i + 1, j + 1);
+        return {1 + length, string(1, s1[i]) + sub};
     }
-    return max(solve(s1, s2, i, j + 1), solve(s1, s2, i + 1, j));
+
+    auto [length1, sub1] = solve(s1, s2, i + 1, j);
+    auto [length2, sub2] = solve(s1, s2, i, j + 1);
+    if (length1 > length2) {
+        return {length1, sub1};
+    }
+    return {length2, sub2};
 }
 
 int main()
@@ -21,7 +28,9 @@ int main()
     string s1, s2;
     cin >> s1 >> s2;
 
-    cout << solve(s1, s2) << endl;
+    auto [size, sub] = solve(s1, s2);
+    cout << size << endl;
+    cout << sub << endl;
 
     return 0;
 }

longest-common-subsequence/src/ac.cpp

@@ -2,25 +2,50 @@
 
 using namespace std;
 
-int memo[1011][1011];
-
-int solve(string &s1, string &s2, int i = 0, int j = 0)
+int main()
 {
-   if (i == s1.size() || j == s2.size()) return 0;
-   if (memo[i][j] != - 1) return memo[i][j];
-   
-   if (s1[i] == s2[j]) {
-      return memo[i][j] = (solve(s1, s2, i + 1, j + 1) + 1);
+   int n, m;
+   cin >> n >> m;
+   string s1, s2;
+   cin >> s1 >> s2;
+
+   /*
+      let dp[i][j] represent the length of the longest common subsequence using the first i characters
+      of s1 and the first j characters of s2
+
+      if s1[i] == s2[j]
+         dp[i][j] = dp[i - 1][j - 1] + 1
+      else
+         dp[i][j] = max(dp[i - 1][j], dp[i][j - 1])
+   */
+
+   vector<vector<int>> dp(n + 1, vector<int>(m + 1, 0));
+   for (int i = 1; i <= n; i++) {
+      for (int j = 1; j <= m; j++) {
+         if (s1[i - 1] == s2[j - 1]) {
+            dp[i][j] = 1 + dp[i - 1][j - 1];
+         } else {
+            dp[i][j] = max(dp[i - 1][j], dp[i][j - 1]);
+         }
+      }
    }
-   return memo[i][j] = max(solve(s1, s2, i, j + 1), solve(s1, s2, i + 1, j));
-}
 
-int main(){
-   int n, m; cin >> n >> m;
-   string s1, s2; cin >> s1 >> s2;
-   memset(memo, -1, sizeof(memo));
-
-   cout << solve(s1, s2) << endl;
+   int row = n, col = m;
+   string sub = "";
+   while (dp[row][col]) {
+      if (s1[row - 1] == s2[col - 1]) {
+         sub = string(1, s1[row - 1]) + sub;
+         row--, col--;
+      } else if (dp[row - 1][col] > dp[row][col - 1]) {
+         row--;
+      } else {
+         col--;
+      }
+   }
+   
+   int size = dp[n][m];
+   cout << size << endl;
+   cout << sub << endl;
 
    return 0;
 }

longest-common-subsequence/src/checker.cpp

@@ -3,15 +3,48 @@
 
 using namespace std;
 
+bool isSubsequence(string &original, string &sub) {
+    int j = 0;
+    for (int i = 0; i < original.size() && j < sub.size(); i++) {
+        if (original[i] == sub[j]) j++;
+    }
+    return j == sub.size();
+}
+
+int readAns(InStream &stream, string &s1, string &s2) {
+    int ans = stream.readInt(0, min(s1.size(), s2.size()), "Common subsequence length");
+    stream.readEoln();
+    string subsequence;
+    if (ans == 0) {
+        stream.readEoln();
+        subsequence = "";
+    } else {
+        subsequence = stream.readToken("[a-z]+");
+        stream.readEoln();
+    }
+    stream.readEof();
+    
+    quitif(subsequence.size() != ans, _wa, "sequence has length different from informed");
+    quitif(!isSubsequence(s1, subsequence), _wa, "%s is not a subsequence of %s", subsequence, s1);
+    quitif(!isSubsequence(s2, subsequence), _wa, "%s is not a subsequence of %s", subsequence, s2);
+
+    return ans;
+}
+
 int main(int argc, char* argv[]) {
-    setName("compare two signed int%d's", 8 * int(sizeof(int)));
+    setName("compare two signed int%d's and check if subsequence given is valid", 8 * int(sizeof(int)));
     registerTestlibCmd(argc, argv);
 
-    int ja = ans.readInt();
-    int pa = ouf.readInt();
+    int n = inf.readInt(), m = inf.readInt();
+    string s1 = inf.readToken(), s2 = inf.readToken();
 
-    if (ja != pa)
-        quitf(_wa, "expected %d, found %d", ja, pa);
+    int ja = readAns(ans, s1, s2);
+    int pa = readAns(ouf, s1, s2);
 
-    quitf(_ok, "all answers are correct");
+    if (ja > pa)
+        quitf(_wa, "jury has the better answer: jans = %d, pans = %d\n", ja, pa);
+    else if (ja < pa)
+        quitf(_fail, "participant has the better answer: jans = %d, pans = %d\n", ja, pa);
+
+    quitf(_ok, "answer is correct");
 }