51. N-Queens (java)

Author: bluesword12350

solution/0051.N-Queens/Solution.java

@@ -0,0 +1,39 @@
+class Solution {
+    private List<List<String>> solutions;
+    private char[][] nQueens;
+    private boolean[] colUsed;
+    private boolean[] diagonals45Used;
+    private boolean[] diagonals135Used;
+    private int n;
+
+    public List<List<String>> solveNQueens(int n) {
+        solutions = new ArrayList<>();
+        nQueens = new char[n][n];
+        for (int i = 0; i < n; i++) Arrays.fill(nQueens[i], '.');
+        colUsed = new boolean[n];
+        diagonals45Used = new boolean[(2 * n) - 1];
+        diagonals135Used = new boolean[(2 * n) - 1];
+        this.n = n;
+        backtracking(0);
+        return solutions;
+    }
+
+    private void backtracking(int row) {
+        if (row == n) {
+            List<String> list = new ArrayList<>();
+            for (char[] chars : nQueens) list.add(new String(chars));
+            solutions.add(list);
+            return;
+        }
+        for (int col = 0; col < n; col++) {
+            int diagonals45Idx = row + col;
+            int diagonals135Idx = n - 1 - (row - col);
+            if (colUsed[col] || diagonals45Used[diagonals45Idx] || diagonals135Used[diagonals135Idx]) continue;
+            nQueens[row][col] = 'Q';
+            colUsed[col] = diagonals45Used[diagonals45Idx] = diagonals135Used[diagonals135Idx] = true;
+            backtracking(row + 1);
+            colUsed[col] = diagonals45Used[diagonals45Idx] = diagonals135Used[diagonals135Idx] = false;
+            nQueens[row][col] = '.';
+        }
+    }
+}