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Commitd002bfe

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author
applewjg
committed
N-Queens
Change-Id: I73c0cd91109752c69f790ed4aa71fa3c924555bc
1 parentdc8725f commitd002bfe

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‎N-Queens.java

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

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