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<h3 id="2">2. &nbsp; 列与对角线剪枝<a class="headerlink" href="#2" title="Permanent link">&para;</a></h3>
<p>为了满足列约束,我们可以利用一个长度为 <span class="arithmatex">\(n\)</span> 的布尔型数组 <code>cols</code> 记录每一列是否有皇后。在每次决定放置前,我们通过 <code>cols</code> 将已有皇后的列进行剪枝,并在回溯中动态更新 <code>cols</code> 的状态。</p>
<p>那么,如何处理对角线约束呢?设棋盘中某个格子的行列索引为 <span class="arithmatex">\((row, col)\)</span> ,选定矩阵中的某条主对角线,我们发现该对角线上所有格子的行索引减列索引都相等,<strong>即对角线上所有格子的 <span class="arithmatex">\(row - col\)</span> 为恒定值</strong></p>
<p>也就是说,如果两个格子满足 <span class="arithmatex">\(row_1 - col_1 = row_2 - col_2\)</span> ,则它们一定处在同一条主对角线上。利用该规律,我们可以借助图 13-18 所示的数组 <code>diag1</code> ,记录每条主对角线上是否有皇后。</p>
<p>同理,<strong>次对角线上的所有格子的 <span class="arithmatex">\(row + col\)</span> 是恒定值</strong>。我们同样也可以借助数组 <code>diag2</code> 来处理次对角线约束。</p>
<p>也就是说,如果两个格子满足 <span class="arithmatex">\(row_1 - col_1 = row_2 - col_2\)</span> ,则它们一定处在同一条主对角线上。利用该规律,我们可以借助图 13-18 所示的数组 <code>diags1</code> ,记录每条主对角线上是否有皇后。</p>
<p>同理,<strong>次对角线上的所有格子的 <span class="arithmatex">\(row + col\)</span> 是恒定值</strong>。我们同样也可以借助数组 <code>diags2</code> 来处理次对角线约束。</p>
<p><a class="glightbox" href="../n_queens_problem.assets/n_queens_cols_diagonals.png" data-type="image" data-width="100%" data-height="auto" data-desc-position="bottom"><img alt="处理列约束和对角线约束" src="../n_queens_problem.assets/n_queens_cols_diagonals.png" /></a></p>
<p align="center"> 图 13-18 &nbsp; 处理列约束和对角线约束 </p>
<h3 id="3">3. &nbsp; 代码实现<a class="headerlink" href="#3" title="Permanent link">&para;</a></h3>
<p>请注意,<span class="arithmatex">\(n\)</span> 维方阵中 <span class="arithmatex">\(row - col\)</span> 的范围是 <span class="arithmatex">\([-n + 1, n - 1]\)</span> <span class="arithmatex">\(row + col\)</span> 的范围是 <span class="arithmatex">\([0, 2n - 2]\)</span> ,所以主对角线和次对角线的数量都为 <span class="arithmatex">\(2n - 1\)</span> ,即数组 <code>diag1</code><code>diag2</code> 的长度都为 <span class="arithmatex">\(2n - 1\)</span></p>
<p>请注意,<span class="arithmatex">\(n\)</span> 维方阵中 <span class="arithmatex">\(row - col\)</span> 的范围是 <span class="arithmatex">\([-n + 1, n - 1]\)</span> <span class="arithmatex">\(row + col\)</span> 的范围是 <span class="arithmatex">\([0, 2n - 2]\)</span> ,所以主对角线和次对角线的数量都为 <span class="arithmatex">\(2n - 1\)</span> ,即数组 <code>diags1</code><code>diags2</code> 的长度都为 <span class="arithmatex">\(2n - 1\)</span></p>
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