Added recursion solutions: Regular Expression Matching and N-Queens

C++/N-queens.cpp

@@ -0,0 +1,44 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+class Solution {
+public:
+    vector<vector<string>> result;
+
+    void solve(int n, int row, vector<string> &board,
+               vector<bool> &cols, vector<bool> &d1, vector<bool> &d2) {
+        if (row == n) {
+            result.push_back(board);
+            return;
+        }
+
+        for (int col = 0; col < n; col++) {
+            if (cols[col] || d1[row - col + n - 1] || d2[row + col]) continue;
+
+            board[row][col] = 'Q';
+            cols[col] = d1[row - col + n - 1] = d2[row + col] = true;
+
+            solve(n, row + 1, board, cols, d1, d2);
+
+            board[row][col] = '.';
+            cols[col] = d1[row - col + n - 1] = d2[row + col] = false;
+        }
+    }
+
+    vector<vector<string>> solveNQueens(int n) {
+        vector<string> board(n, string(n, '.'));
+        vector<bool> cols(n), d1(2*n - 1), d2(2*n - 1);
+        solve(n, 0, board, cols, d1, d2);
+        return result;
+    }
+};
+
+int main() {
+    Solution sol;
+    auto ans = sol.solveNQueens(4);
+    for (auto &board : ans) {
+        for (auto &row : board)
+            cout << row << endl;
+        cout << "------" << endl;
+    }
+}

C++/regular-expression-matching.cpp

@@ -0,0 +1,25 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+class Solution {
+public:
+    bool isMatch(string s, string p) {
+        if (p.empty()) return s.empty();
+
+        bool firstMatch = (!s.empty() && (s[0] == p[0] || p[0] == '.'));
+
+        if (p.size() >= 2 && p[1] == '*') {
+            // Two options: skip the pattern or use it if matches
+            return (isMatch(s, p.substr(2)) || 
+                   (firstMatch && isMatch(s.substr(1), p)));
+        } 
+        else {
+            return firstMatch && isMatch(s.substr(1), p.substr(1));
+        }
+    }
+};
+
+int main() {
+    Solution sol;
+    cout << boolalpha << sol.isMatch("aab", "c*a*b") << endl; // true
+}