JavaScript/Backtracking/NQueen.js

class NQueen {
  constructor (size) {
    this.board = new Array(size).fill('.').map(() => new Array(size).fill('.'))
    this.size = size
  }

  isValid ([row, col]) {
    // function to check if the placement of the queen in the given location is valid

    // checking the left of the current row
    for (let i = 0; i < col; i++) {
      if (this.board[row][i] === 'Q') return false
    }

    // checking the upper left diagonal
    for (let i = row, j = col; i >= 0 && j >= 0; i--, j--) {
      if (this.board[i][j] === 'Q') return false
    }

    // checking the lower left diagonal
    for (let i = row, j = col; j >= 0 && i < this.size; i++, j--) {
      if (this.board[i][j] === 'Q') return false
    }

    return true
  }

  solve (col = 0) {
    // function to solve the board
    if (col >= this.size) { return true }

    for (let i = 0; i < this.size; i++) {
      if (this.isValid([i, col])) {
        this.board[i][col] = 'Q'

        if (this.solve(col + 1)) { return true }

        // backtracking
        this.board[i][col] = '.'
      }
    }

    return false
  }

  printBoard () {
    // utility function to display the board
    for (const row of this.board) {
      console.log(...row)
    }
  }
}

function main () {
  const board = new NQueen(8)

  board.printBoard()
  console.log('\n')

  board.solve()

  board.printBoard()
}

main()