N-Queens - Implementation

Here is the implementation:

function solveNQueens(n):
    result = []
    board = n x n grid of '.'
    cols = set(), diag1 = set(), diag2 = set()
    backtrack(0)
    return result

function backtrack(row):
    if row == n:
        result.append(board as string array)
        return

    for col from 0 to n - 1:
        if col in cols or (row - col) in diag1 or (row + col) in diag2:
            continue

        board[row][col] = 'Q'
        cols.add(col)
        diag1.add(row - col)
        diag2.add(row + col)

        backtrack(row + 1)

        board[row][col] = '.'
        cols.remove(col)
        diag1.remove(row - col)
        diag2.remove(row + col)

Time is $O(n!)$ with pruning. Space is $O(n)$ for sets and recursion.