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Commit 18ba3a4

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juffalowtrekhleb
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Add N queens problem bitwise solution (trekhleb#15)
* Add N queens problem bitwise solution * Update code to corespond with eslint
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# N-Queens Problem with Bitwise Solution
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Write a function that will find all possible solutions to the N queens problem for a given N.
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## References
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- [Wikipedia](https://en.wikipedia.org/wiki/Eight_queens_puzzle)
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- [GREG TROWBRIDGE](http://gregtrowbridge.com/a-bitwise-solution-to-the-n-queens-problem-in-javascript/)
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- [Backtracking Algorithms in MCPL using Bit Patterns and Recursion](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.51.7113&rep=rep1&type=pdf)
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import nQueensBitwise from '../nQueensBitwise';
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describe('nQueensBitwise', () => {
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it('should have solutions for 4 to N queens', () => {
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const solutionFor4 = nQueensBitwise(4);
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expect(solutionFor4).toBe(2);
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const solutionFor5 = nQueensBitwise(5);
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expect(solutionFor5).toBe(10);
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const solutionFor6 = nQueensBitwise(6);
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expect(solutionFor6).toBe(4);
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const solutionFor7 = nQueensBitwise(7);
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expect(solutionFor7).toBe(40);
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const solutionFor8 = nQueensBitwise(8);
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expect(solutionFor8).toBe(92);
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const solutionFor9 = nQueensBitwise(9);
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expect(solutionFor9).toBe(352);
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const solutionFor10 = nQueensBitwise(10);
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expect(solutionFor10).toBe(724);
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});
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});
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export default function (n) {
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// Keeps track of the # of valid solutions
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let count = 0;
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// Helps identify valid solutions
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const done = (2 ** n) - 1;
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// Checks all possible board configurations
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const innerRecurse = (ld, col, rd) => {
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// All columns are occupied,
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// so the solution must be complete
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if (col === done) {
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count += 1;
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return;
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}
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// Gets a bit sequence with "1"s
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// whereever there is an open "slot"
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let poss = ~(ld | rd | col);
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// Loops as long as there is a valid
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// place to put another queen.
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while (poss & done) {
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const bit = poss & -poss;
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poss -= bit;
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innerRecurse((ld | bit) >> 1, col | bit, (rd | bit) << 1);
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}
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};
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innerRecurse(0, 0, 0);
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return count;
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}

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