From cff768e63ccf95ba1433c874d31d781028e7d98c Mon Sep 17 00:00:00 2001
From: Hanlin Shi <shihanlin9@gmail.com>
Date: Mon, 31 May 2021 00:43:43 -0700
Subject: [PATCH] Add solution for N-Queen problem which uses bit-operation

Still uses DFS, optimized for state compression.

Signed-off-by: Hanlin Shi <shihanlin9@gmail.com>
---
 leetcode/0051.N-Queens/51. N-Queens.go | 80 +++++++++++++++-----------
 1 file changed, 45 insertions(+), 35 deletions(-)

diff --git a/leetcode/0051.N-Queens/51. N-Queens.go b/leetcode/0051.N-Queens/51. N-Queens.go
index de68f47b..040718dd 100644
--- a/leetcode/0051.N-Queens/51. N-Queens.go	
+++ b/leetcode/0051.N-Queens/51. N-Queens.go	
@@ -48,38 +48,48 @@ func generateBoard(n int, row *[]int) []string {
 }
 
 // 解法二 二进制操作法
-// class Solution
-// {
-//     int n;
-//     string getNq(int p)
-//     {
-//         string s(n, '.');
-//         s[p] = 'Q';
-//         return s;
-//     }
-//     void nQueens(int p, int l, int m, int r, vector<vector<string>> &res)
-//     {
-//         static vector<string> ans;
-//         if (p >= n)
-//         {
-//             res.push_back(ans);
-//             return ;
-//         }
-//         int mask = l | m | r;
-//         for (int i = 0, b = 1; i < n; ++ i, b <<= 1)
-//             if (!(mask & b))
-//             {
-//                 ans.push_back(getNq(i));
-//                 nQueens(p + 1, (l | b) >> 1, m | b, (r | b) << 1, res);
-//                 ans.pop_back();
-//             }
-//     }
-// public:
-//     vector<vector<string> > solveNQueens(int n)
-//     {
-//         this->n = n;
-//         vector<vector<string>> res;
-//         nQueens(0, 0, 0, 0, res);
-//         return res;
-//     }
-// };
+func solveNQueens2(n int) (res [][]string) {
+	placements := make([]string, n)
+	for i := range placements {
+		buf := make([]byte, n)
+		for j := range placements {
+			if i == j {
+				buf[j] = 'Q'
+			} else {
+				buf[j] = '.'
+			}
+		}
+		placements[i] = string(buf)
+	}
+
+	var construct func(prev []int)
+	construct = func(prev []int) {
+		if len(prev) == n {
+			plan := make([]string, n)
+			for i := 0; i < n; i++ {
+				plan[i] = placements[prev[i]]
+			}
+			res = append(res, plan)
+			return
+		}
+
+		occupied := 0
+		for i := range prev {
+			dist := len(prev) - i
+			bit := 1 << prev[i]
+			occupied |= bit | bit<<dist | bit>>dist
+		}
+
+		prev = append(prev, -1)
+		for i := 0; i < n; i++ {
+			if (occupied>>i)&1 != 0 {
+				continue
+			}
+			prev[len(prev)-1] = i
+			construct(prev)
+		}
+	}
+
+	construct(make([]int, 0, n))
+	return
+}