添加 0052.N皇后II.md Java版本

problems/0052.N皇后II.md

@@ -256,7 +256,54 @@ int totalNQueens(int n){
     return answer;
 }
 ```
-
+Java
+```java
+class Solution {
+    int count = 0;
+    public int totalNQueens(int n) {
+        char[][] board = new char[n][n];
+        for (char[] chars : board) {
+            Arrays.fill(chars, '.');
+        }
+        backTrack(n, 0, board);
+        return count;
+    }
+    private void backTrack(int n, int row, char[][] board) {
+        if (row == n) {
+            count++;
+            return;
+        }
+        for (int col = 0; col < n; col++) {
+            if (isValid(row, col, n, board)) {
+                board[row][col] = 'Q';
+                backTrack(n, row + 1, board);
+                board[row][col] = '.';
+            }
+        }
+    }
+    private boolean isValid(int row, int col, int n, char[][] board) {
+        // 检查列
+        for (int i = 0; i < row; ++i) {
+            if (board[i][col] == 'Q') {
+                return false;
+            }
+        }
+        // 检查45度对角线
+        for (int i = row - 1, j = col - 1; i >= 0 && j >= 0; i--, j--) {
+            if (board[i][j] == 'Q') {
+                return false;
+            }
+        }
+        // 检查135度对角线
+        for (int i = row - 1, j = col + 1; i >= 0 && j <= n - 1; i--, j++) {
+            if (board[i][j] == 'Q') {
+                return false;
+            }
+        }
+        return true;
+    }
+}
+```
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