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每日1题202010/[1017]52.n皇后-ii_回溯剪枝.php

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+<?php
+/*
+ * @lc app=leetcode.cn id=52 lang=php
+ *
+ * [52] N皇后 II
+ * 
+ * https://leetcode-cn.com/problems/n-queens-ii/
+ * 参考题解：https://leetcode-cn.com/problems/n-queens/solution/hui-su-fa-phpjian-ji-xie-fa-by-jasper-33/
+ */
+
+// @lc code=start
+class Solution {
+
+    /**
+     * @param Integer $n
+     * @return Integer
+     */
+    function totalNQueens($n) {
+
+        if($n < 1) {
+            return 0;
+        }
+
+        // 存放皇后攻击的列、左对角线、右对角线
+        $this->cols = [];
+        $this->pie = [];
+        $this->na = [];
+
+        $res = [];
+        $this->dfs($n, 0, [], $res);
+
+        return count($res);
+    }
+
+    function dfs($n, $row, $cur, &$res) {
+
+        if($row == $n){
+            $res[] = $cur;
+            return true;
+        }
+
+        for ($col=0; $col < $n; $col++) { 
+
+            // 剪枝（当前列、左、右对角线已存在，跳过）
+            if(in_array($col, $this->cols) || in_array($row + $col, $this->pie) || in_array($row - $col, $this->na)) {
+                continue;
+            }
+
+            $this->cols[] = $col;
+            $this->pie[] = $row + $col; // 左对角线的值一致
+            $this->na[] = $row - $col; // 右对角线的值一致
+
+            // $cur 不能受当前层的影响
+            $this->dfs($n, $row + 1, array_merge($cur, [$col]), $res);
+
+            array_pop($this->cols);
+            array_pop($this->pie);
+            array_pop($this->na);
+        }
+    }
+}
+
+// Accepted
+// 9/9 cases passed (20 ms)
+// Your runtime beats 58.11 % of php submissions
+// Your memory usage beats 8.33 % of php submissions (15.4 MB)
+// @lc code=end
+