Create Longest palindromic subsequence - DP.cpp

Author: nikolatechie

Longest palindromic subsequence - DP.cpp

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+// Dynamic programming solution
+#include <bits/stdc++.h>
+#include <ext/rope>
+using namespace std;
+using namespace __gnu_cxx;
+
+using ld = long double;
+using llint = long long;
+using ullint = unsigned long long;
+using pii = pair <int,int>;
+using pcc = pair <char,char>;
+using pss = pair <string,string>;
+using vi = vector <int>;
+using vb = vector <bool>;
+using vii = vi::iterator;
+
+#define INF (1<<30)
+#define MOD 1000000007
+#define mp make_pair
+#define mt make_tuple
+#define all(c) c.begin(), c.end()
+#define ms(name,val) memset(name, val, sizeof name)
+#define np nullptr
+
+
+int n;
+string s;
+vector <vi> DP;
+
+void printLPS()
+{
+    string seq;
+    int i = 0, j = n-1;
+
+    while (i <= j)
+    {
+        if (s[i] == s[j])
+        {
+            seq += s[j];
+            ++i, --j;
+        }
+        else
+        {
+            if (DP[i][j-1] > DP[i+1][j])
+                --j;
+            else
+                ++i;
+        }
+    }
+
+    int m = seq.size();
+
+    if (DP[0][n-1] & 1)
+    {
+        if (m == 1)
+            cout << seq << '\n';
+        else
+        {
+            string add = string(1, seq[m-1]);
+            seq.pop_back();
+            string rseq = seq;
+            reverse(all(rseq));
+            cout << seq << add << rseq << '\n';
+        }
+    }
+    else
+    {
+        string rseq = seq;
+        reverse(all(rseq));
+        cout << seq << rseq << '\n';
+    }
+}
+
+int findLPS()
+{
+    n = s.size();
+    DP.resize(n);
+
+    for (int t1 = 0; t1 < n; ++t1)
+    {
+        DP[t1].resize(n);
+        DP[t1][t1] = 1;
+    }
+
+    for (int topCol = 1; topCol < n; ++topCol)
+    {
+        for (int i = 0, j = topCol; j < n; ++i, ++j)
+        {
+            if (s[i] == s[j])
+            {
+                DP[i][j] = 2; // maximal if len of sequence is 2
+                if (j > i+1) DP[i][j] += DP[i+1][j-1];
+            }
+            else DP[i][j] = max(DP[i][j-1], DP[i+1][j]);
+            // maximum by:
+            // 1) ignoring letter s[i]
+            // 2) ignoring letter s[j]
+        }
+    }
+
+    return DP[0][n-1];
+}
+
+int main()
+{
+    ios_base::sync_with_stdio(0);
+    //cin.tie(0);
+
+    cin >> s;
+    cout << findLPS() << '\n';
+    printLPS();
+
+
+
+    return 0;
+}