N-Queens

Change-Id: I73c0cd91109752c69f790ed4aa71fa3c924555bc

Author: applewjg

N-Queens.java

@@ -0,0 +1,57 @@
+/*
+ Author:     King, [email protected]
+ Date:       Jul 25, 2013
+ Problem:    N-Queens
+ Difficulty: Medium
+ Source:     https://oj.leetcode.com/problems/n-queens/
+ Notes:
+ The n-queens puzzle is the problem of placing n queens on an n*n chessboard such that no two queens attack each other.
+ Given an integer n, return all distinct solutions to the n-queens puzzle.
+ Each solution contains a distinct board configuration of the n-queens' placement, where 'Q' and '.' both indicate a queen and an empty space respectively.
+ For example,
+ There exist two distinct solutions to the 4-queens puzzle:
+ [
+ [".Q..",  // Solution 1
+ "...Q",
+ "Q...",
+ "..Q."],
+
+ ["..Q.",  // Solution 2
+ "Q...",
+ "...Q",
+ ".Q.."]
+ ]
+
+ Solution: Recursion (DFS). Use bit-manipulation solution (See N-QueensII for more details).
+ */
+
+
+public class Solution {
+    public List<String[]> solveNQueens(int n) {
+        List<String[]> res = new ArrayList<String[]>();
+        List<char[]> sol = new ArrayList<char[]>();
+        solveNQueensRe(n, 0, 0, 0, sol, res);
+        return res;
+    }
+    public void solveNQueensRe(int n, int row, int ld, int rd, List<char[]> sol, List<String[]> res) {
+        if (row == (1<<n) -1 ) {
+            String[] temp = new String[n];
+            for (int i = 0; i < n; ++i) 
+                temp[i] = String.valueOf(sol.get(i)); 
+            res.add(temp);
+            return;
+        }
+        int avail = ~(row | ld | rd);
+        for (int i = n -1; i >= 0; --i) {
+            int pos = 1 << i;
+            if ((int)(avail & pos) != 0) {
+                char[] str = new char[n];
+                Arrays.fill(str, '.');
+                str[i] = 'Q';
+                sol.add(str);
+                solveNQueensRe(n, row | pos, (ld|pos)<<1, (rd|pos)>>1, sol, res);
+                sol.remove(sol.size()-1);
+            }
+        }
+    }  
+}