20190409

code/lc51.java

+package code;
+/*
+ * 51. N-Queens
+ * 题意：8皇后问题
+ * 难度：Hard
+ * 分类：Backtracking
+ * 思路：回溯+判断，注意怎么判断两个斜线方向
+ * Tips：
+ */
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+
+public class lc51 {
+    StringBuilder ss = new StringBuilder();
+    HashSet<String> hs = new HashSet();
+    public List<List<String>> solveNQueens(int n) {
+        for (int i = 0; i < n-1 ; i++) {
+            ss.append(".");
+        }
+        return dfs(new ArrayList<>(), new ArrayList(), n, 0);
+    }
+
+    public List<List<String>> dfs(List<List<String>> res, List curr, int n, int row){
+        if(row==n){
+            res.add(new ArrayList<>(curr));
+            return res;
+        }
+        for (int i = 0; i < n ; i++) {
+            if(isValid(row, i)){
+                curr.add(new StringBuilder(ss).insert(i,"Q").toString());
+                int a = row+i;  // row+col 作为key
+                int b = row-i;  // row-col 作为key
+                hs.add("row"+row); hs.add("col"+i); hs.add("k1"+a); hs.add("k2"+b);
+                dfs(res, curr, n, row+1);
+                curr.remove(curr.size()-1);
+                hs.remove("row"+row); hs.remove("col"+i); hs.remove("k1"+a); hs.remove("k2"+b); //别忘了删掉
+            }
+        }
+        return res;
+    }
+
+    public boolean isValid(int row, int col){
+        int a = row+col;
+        int b = row-col;
+        if(hs.contains("row"+row)||hs.contains("col"+col)||hs.contains("k1"+a)||hs.contains("k2"+b)) return false;
+        return true;
+    }
+}

readme.md

@@ -35,10 +35,10 @@ LeetCode 指南
 | 009 [Java](./code/lc9.java)
 | 010 [Java](./code/lc10.java)
 | 011 [Java](./code/lc11.java)
-| 013 [Java](./code/lc13java)
-| 014 [Java](./code/lc14java)
-| 015 [Java](./code/lc15java)
-| 016 [Java](./code/lc16java)
+| 013 [Java](./code/lc13.java)
+| 014 [Java](./code/lc14.java)
+| 015 [Java](./code/lc15.java)
+| 016 [Java](./code/lc16.java)
 | 017 [Java](./code/lc17.java)
 | 019 [Java](./code/lc19.java)
 | 020 [Java](./code/lc20.java)
@@ -66,6 +66,7 @@ LeetCode 指南
 | 048 [Java](./code/lc48.java)
 | 049 [Java](./code/lc49.java)
 | 050 [Java](./code/lc50.java)
+| 051 [Java](./code/lc51.java)
 | 053 [Java](./code/lc53.java)
 | 054 [Java](./code/lc54.java)
 | 055 [Java](./code/lc55.java)
@@ -220,4 +221,8 @@ LeetCode 指南
 | 746 [Java](./code/lc746.java)
 | 771 [Java](./code/lc771.java)
 | 877 [Java](./code/lc877.java)
+| 921 [Java](./code/lc921.java)
+| 922 [Java](./code/lc922.java)
+| 923 [Java](./code/lc923.java)
+| 978 [Java](./code/lc978.java)
 | 983 [Java](./code/lc983.java)