NQueens

Recursion/NQueens.java

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+import java.lang.reflect.Array;
+import java.util.ArrayList;
+import java.util.List;
+
+// complexity is b ^ h ; in here TC is n! * n otherwise n ^ n
+//SC is O(n) + o(n) + n! * n
+
+
+//[[
+//"--q-",
+//"---q",
+//"---q",
+//"-q--"
+//], [
+//"-q--",
+//"---q",
+//"q---",
+//"--q-"
+//]]
+
+public class NQueens {
+    public static List<List<String>> result = new ArrayList<>();
+    public static int counter = 0;
+
+    public static String[][] solveNQueens(int n) {
+
+        List<Integer> board = new ArrayList<>(n);
+        for(int i = 0; i < n; i++)
+            board.add(i);
+        solveNQueensHelper(0, board);
+        String[][] s = new String[result.size()][n];
+        for(int i=0; i <result.size(); i++) {
+            for(int j =0; j <n; j++) {
+                s[i][j]=result.get(i).get(j);
+            }
+        }
+        return s;
+    }
+
+    private static void solveNQueensHelper(int i, List<Integer> board) {
+
+        if (i == board.size()) {
+            counter++;
+            List<String> l = new ArrayList<>();
+
+            for(int k = 0; k < board.size(); k++) {
+                int col = board.get(k);
+                StringBuilder sb = new StringBuilder();
+                for(int e = 0; e < board.size(); e++) {
+                    if(e!= col)
+                        sb.append("-");
+                    else
+                        sb.append("q");
+                }
+                l.add(sb.toString());
+            }
+            result.add(l);
+        }
+
+        for (int j = i; j < board.size(); j++) {
+            swapMe(board, i, j);
+            if(isValid(i, board))
+                solveNQueensHelper(i + 1, board);
+            swapMe(board, i, j);
+        }
+    }
+
+    private static boolean isValid(int i, List<Integer> board) {
+        for (int c = 0; c < i; c++) {
+            int rowDiff = Math.abs(i - c);
+            int colDiff = Math.abs(board.get(i) - board.get(c));
+            if (rowDiff == colDiff) return false;
+        }
+        return true;
+    }
+
+    private static void swapMe(List<Integer> board, int idx1, int idx2) {
+        int temp = board.get(idx1);
+        board.set(idx1,board.get(idx2));
+        board.set(idx2,temp);
+    }
+
+    public static void main(String[] args) {
+        solveNQueens(8);
+        System.out.println(result);
+        System.out.println(counter);
+    }
+}