Merge pull request #3441 from Tetsuya3850/patch-2

Author: felivalencia3

Diff for: java/0052-n-queens-ii.java

@@ -0,0 +1,38 @@
+class Solution {
+    int count = 0;
+
+    public int totalNQueens(int n) {
+        Set<Integer> colSet = new HashSet<>();
+        Set<Integer> posDiagSet = new HashSet<>(); // (r + c)
+        Set<Integer> negDiagSet = new HashSet<>(); // (r - c)
+        backtrack(0, n, colSet, posDiagSet, negDiagSet);
+        return count;
+    }
+
+    private void backtrack(
+            int row,
+            int n,
+            Set<Integer> colSet,
+            Set<Integer> posDiagSet,
+            Set<Integer> negDiagSet) {
+        if (row == n) {
+            count += 1;
+            return;
+        }
+
+        for (int col = 0; col < n; col++) {
+            if (colSet.contains(col)
+                    || posDiagSet.contains(row + col)
+                    || negDiagSet.contains(row - col)) {
+                continue;
+            }
+            colSet.add(col);
+            posDiagSet.add(row + col);
+            negDiagSet.add(row - col);
+            backtrack(row + 1, n, colSet, posDiagSet, negDiagSet);
+            colSet.remove(col);
+            posDiagSet.remove(row + col);
+            negDiagSet.remove(row - col);
+        }
+    }
+}