Merge branch 'main' into lcque

Author: SjxSubham

51.N-Queens.cpp

@@ -0,0 +1,44 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+class Solution {
+public:
+    void solve(int col, int n, vector<string> &board, 
+               vector<vector<string>> &ans, 
+               vector<int> &leftrow, vector<int> &upperdiag, vector<int> &lowerdiag) {
+        
+        if (col == n) {
+            ans.push_back(board);
+            return;
+        }
+
+        for (int row = 0; row < n; row++) {
+            if (leftrow[row] == 0 && lowerdiag[row + col] == 0 && upperdiag[(n - 1) + (col - row)] == 0) {
+                
+                board[row][col] = 'Q';
+                leftrow[row] = 1;
+                lowerdiag[row + col] = 1;
+                upperdiag[(n - 1) + (col - row)] = 1;
+
+                solve(col + 1, n, board, ans, leftrow, upperdiag, lowerdiag);
+
+                board[row][col] = '.';  // backtrack
+                leftrow[row] = 0;
+                lowerdiag[row + col] = 0;
+                upperdiag[(n - 1) + (col - row)] = 0;
+            }
+        }
+    }
+
+    vector<vector<string>> solveNQueens(int n) {
+        vector<vector<string>> ans;
+        vector<string> board(n, string(n, '.'));
+
+        vector<int> leftrow(n, 0);
+        vector<int> lowerdiag(2 * n - 1, 0);
+        vector<int> upperdiag(2 * n - 1, 0);
+
+        solve(0, n, board, ans, leftrow, upperdiag, lowerdiag);
+        return ans;
+    }
+};