# Backtracking — N Queens Problem — Better Solution

Objec­tive : In chess, a queen can move as far as she pleases, hor­i­zon­tally, ver­ti­cally, or diag­o­nally. A chess board has 8 rows and 8 columns. The stan­dard 8 by 8 Queen’s prob­lem asks how to place 8 queens on an ordi­nary chess board so that none of them can hit any other in one move.(Source: http://www.math.utah.edu/~alfeld/queens/queens.html)

Here we are solv­ing it for N queens in NxN chess board.

N Queens Problem

Approach:

This ear­lier approach we have seen solu­tion matrix, at every row we have only one entry as 1 and rest of the entries are 0. Solu­tion matrix takes O(N2) space. We can reduce it to O(N). We will solve it by tak­ing one dimen­sional array and con­sider solution[1] = 2 as “Queen at 1st row is placed at 2nd column.

result[i]=j means queen at i-th row is placed at j-th column.

Check if Queens placed at (x1, y1) and (x2,y2) are safe

• x1==x2 means same rows,
• y1==y2 means same columns
• |x2-x1|==|y2-y1| means they are placed in diagonals.

Code:

```Output:
Order of 6 queens[1, 3, 5, 0, 2, 4]
Order of 6 queens[2, 5, 1, 4, 0, 3]
Order of 6 queens[3, 0, 4, 1, 5, 2]
Order of 6 queens[4, 2, 0, 5, 3, 1]

```

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• Car­los de la Torre

Ok, I see this is an improved solu­tion. Still back­track­ing. Check this out: https://thewalnut.io/simulations/edit_agent/94/
A com­pletely dif­fer­ent approach. Very inter­est­ing indeed.

• Paras

It will be great if you post solu­tion in cpp rather than JAVA in future :/

• Tạ Anh Tú

Java is still great