Added recursion solutions: Regular Expression Matching and N-Queens

Author: avimishraa

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
+}