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Commitaed6fa2

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author
applewjg
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N-Queen II
Change-Id: I0cea47edcf32d09b399de1f4b946c67b17f69e17
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‎N-QueensII.java

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/*
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Author: King, nkuwjg@gmail.com
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Date: Aug 23, 2013
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Problem: N-Queens II
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Difficulty: Medium
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Source: https://oj.leetcode.com/problems/n-queens-ii/
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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: 1. Recursion.
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2. Recursion + bit version. (fast)
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The idea is from http://www.matrix67.com/blog/archives/266 (in chinese).
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*/
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publicclassSolution {
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publicinttotalNQueens(intn) {
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returntotalNQueens_2(n);
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}
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publicinttotalNQueens_1(intn) {
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int[]board =newint[n];
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Arrays.fill(board,-1);
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int[]res =newint[1];
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totalNQueensRe(n,0,board,res);
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returnres[0];
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}
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publicvoidtotalNQueensRe(intn,introw,int[]board,int[]res) {
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if (n ==row) {
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res[0]++;
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return;
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}
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for (inti =0;i <n; ++i) {
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if (isValid(board,row,i)) {
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board[row] =i;
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totalNQueensRe(n,row +1,board,res);
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board[row] = -1;
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}
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}
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}
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publicbooleanisValid(int[]board,introw,intcol) {
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for (inti =0;i <row; ++i) {
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if (board[i] ==col ||row -i ==Math.abs(col -board[i]))
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returnfalse;
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}
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returntrue;
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}
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publicinttotalNQueens_2(intn) {
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int[]res =newint[1];
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totalNQueensRe2(n,0,0,0,res);
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returnres[0];
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}
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publicvoidtotalNQueensRe2(intn,introw,intld,intrd,int[]res) {
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if (row == (1<<n) -1 ) {
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res[0]++;
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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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totalNQueensRe2(n,row |pos, (ld|pos) <<1, (rd|pos) >>1,res);
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}
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}
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}
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}

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