From a490c4b4fd7d3a2d22dfe2e20bb27dc55cd9d237 Mon Sep 17 00:00:00 2001
From: GitNaruto <19946254539@163.com>
Date: Fri, 25 Nov 2022 15:00:03 +0800
Subject: [PATCH] =?UTF-8?q?=E6=B7=BB=E5=8A=A00051N=E7=9A=87=E5=90=8EJava?=
 =?UTF-8?q?=E7=89=88=E6=9C=AC(=E6=96=B9=E6=B3=952)?=
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---
 problems/0051.N皇后.md | 50 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/problems/0051.N皇后.md b/problems/0051.N皇后.md
index 83d0cd94..16fdf347 100644
--- a/problems/0051.N皇后.md
+++ b/problems/0051.N皇后.md
@@ -291,6 +291,56 @@ class Solution {
 }
 ```
 
+```java
+// 方法2：使用boolean数组表示已经占用的直(斜)线
+class Solution {
+    List<List<String>> res = new ArrayList<>();
+    boolean[] usedCol, usedDiag45, usedDiag135;    // boolean数组中的每个元素代表一条直(斜)线
+    public List<List<String>> solveNQueens(int n) {
+        usedCol = new boolean[n];                  // 列方向的直线条数为 n
+        usedDiag45 = new boolean[2 * n - 1];       // 45°方向的斜线条数为 2 * n - 1
+        usedDiag135 = new boolean[2 * n - 1];      // 135°方向的斜线条数为 2 * n - 1
+		//用于收集结果, 元素的index表示棋盘的row，元素的value代表棋盘的column
+        int[] board = new int[n];
+        backTracking(board, n, 0);
+        return res;
+    }
+    private void backTracking(int[] board, int n, int row) {
+        if (row == n) {
+            //收集结果
+            List<String> temp = new ArrayList<>();
+            for (int i : board) {
+                char[] str = new char[n];
+                Arrays.fill(str, '.');
+                str[i] = 'Q';
+                temp.add(new String(str));
+            }
+            res.add(temp);
+            return;
+        }
+
+        for (int col = 0; col < n; col++) {
+            if (usedCol[col] | usedDiag45[row + col] | usedDiag135[row - col + n - 1]) {
+                continue;
+            }
+            board[row] = col;
+			// 标记该列出现过
+            usedCol[col] = true;
+			// 同一45°斜线上元素的row + col为定值, 且各不相同
+            usedDiag45[row + col] = true;
+			// 同一135°斜线上元素row - col为定值, 且各不相同
+			// row - col 值有正有负, 加 n - 1 是为了对齐零点
+            usedDiag135[row - col + n - 1] = true;
+            // 递归
+            backTracking(board, n, row + 1);
+            usedCol[col] = false;
+            usedDiag45[row + col] = false;
+            usedDiag135[row - col + n - 1] = false;
+        }
+    }
+}
+```
+
 ### Python 
 
 ```python