In this tutorial, you will learn about Backtracking and its approach to solve constraint satisfaction problems such as crosswords and puzzles

### What is Backtracking?

Backtracking is a method for solving a constraint satisfaction problem by incrementally building candidates to solution and abandoning a candidate as soon as it can not satisfy the constraint

Let's see a specific example

### The N queens puzzle

Write an algorithm to place N chess queens on an NxN chessboard so that no two queens threaten each other; thus, a solution requires that no two queens share the same row, column or diagonal

### Backtracking approach

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`int[][] board = new int[8][8]` all cells default value are 0

`isValid(board, row, col)` ensures no two queens share the same row, column or diagonal

`enumerate(board, col)` tries all possible candidates starting from column 0 to 7, backtracks to remove the queen from the board as soon as can not build solution

`col == board.length` means all queen are placed on the board

`board[i][col] = 1` places a queen on the board at rows `i` and column `col`

`board[i][col] = 0` removes the queen at rows `i` and column `col` from the board