Added solution for Leetcode - 51

N-Queens.cpp

@@ -0,0 +1,75 @@
+class Solution {
+private:
+    vector<vector<string>> Help(int n) {
+        vector<vector<string>> ans;
+        vector<vector<int>> board(n, vector<int>(n,0));
+        solve(0,board,ans,n);
+        return ans;
+    }
+
+    bool isSafe(int r, int c, vector<vector<int>> &board, int n){
+        int x = r;
+        int y = c;
+
+        // check for column
+        while(x >= 0){
+            if(board[x][y] == 1) return false;
+            x--;
+        }
+
+        x = r, y = c;
+        // check one diagonal
+        if(x>0){
+            while( x>=0 && y<n){
+                if(board[x][y] == 1) return false;
+                x--;
+                y++;
+            }
+        }
+
+        x = r, y = c;
+        // check another diagonal
+        if(y>0){
+            while( x>=0 && y>=0){
+                if(board[x][y] == 1) return false;
+                x--;
+                y--;
+            }
+        }
+
+        return true;
+    }
+private:
+    void solve(int row,  vector<vector<int>> &board,  vector<vector<string>> &ans, int n){
+        // base case
+        if(row == n){
+            vector<string> temp;
+            for(int i=0; i<n; i++){
+                string s="";
+                for(int j=0; j<n; j++){
+                    if(board[i][j] == 1) s.push_back('Q');
+                    else s.push_back('.');
+                }
+                temp.push_back(s);
+            }
+            ans.push_back(temp);
+            return;
+        }
+
+        for(int col=0; col<n; col++){
+            if(isSafe(row,col,board,n)){
+                board[row][col] = 1;
+                solve(row+1, board, ans, n);
+                board[row][col] = 0;
+            }
+        }
+
+    }
+public:
+    vector<vector<string>> solveNQueens(int n) {
+        vector<vector<string>> ans;
+        vector<vector<int>> board(n, vector<int>(n,0));
+        solve(0,board,ans,n);
+        return ans;
+    }
+};