content: package codingquestions.leetcode;
  
  /*
  37. Sudoku Solver
  Solved
  Hard
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  Write a program to solve a Sudoku puzzle by filling the empty cells.
  
  A sudoku solution must satisfy all of the following rules:
  
  Each of the digits 1-9 must occur exactly once in each row.
  Each of the digits 1-9 must occur exactly once in each column.
  Each of the digits 1-9 must occur exactly once in each of the 9 3x3 sub-boxes of the grid.
  The '.' character indicates empty cells.
  
   
  
  Example 1:
  
  
  Input: board = [["5","3",".",".","7",".",".",".","."],["6",".",".","1","9","5",".",".","."],[".","9","8",".",".",".",".","6","."],["8",".",".",".","6",".",".",".","3"],["4",".",".","8",".","3",".",".","1"],["7",".",".",".","2",".",".",".","6"],[".","6",".",".",".",".","2","8","."],[".",".",".","4","1","9",".",".","5"],[".",".",".",".","8",".",".","7","9"]]
  Output: [["5","3","4","6","7","8","9","1","2"],["6","7","2","1","9","5","3","4","8"],["1","9","8","3","4","2","5","6","7"],["8","5","9","7","6","1","4","2","3"],["4","2","6","8","5","3","7","9","1"],["7","1","3","9","2","4","8","5","6"],["9","6","1","5","3","7","2","8","4"],["2","8","7","4","1","9","6","3","5"],["3","4","5","2","8","6","1","7","9"]]
  Explanation: The input board is shown above and the only valid solution is shown below:
  
  
   
  
  Constraints:
  
  board.length == 9
  board[i].length == 9
  board[i][j] is a digit or '.'.
  It is guaranteed that the input board has only one solution.
  */
  class SudokuSolver {
      public char[][] c = new char[9][9];
      public boolean canBePlaced(char cc, int row, int col){
          //check the same row(
          for(int i = 0; i < 9; i++){
              if(c[row][i] == cc) return false;
          }
  
          //same col
          for(int i = 0; i < 9; i++){
              if(c[i][col] == cc) return false;
          }
  
          //same square
          int startRow = ((int)(row / 3)) * 3, startCol = ((int)(col / 3) * 3);
          for(int i = 0; i < 3; i++){
              for(int j = 0; j < 3; j++){
                  if(c[startRow + i][startCol + j] == cc) return false;
              }
          }
          return true;
      }
  
      public boolean solveHelper(int row, int col){
          if(col == 9){
              row++;
              col = 0;
          }
          if(row == 9) return true;
  
          if(c[row][col] != '.') return solveHelper(row, col + 1);
  
          for(char num = '1'; num <= '9'; num++){
              if(canBePlaced(num, row, col)){
                  c[row][col] = num;
                  if(solveHelper(row, col + 1)) return true;
  
                  c[row][col] = '.';
              }
          }
          return false;
      }
  
      public void solveSudoku(char[][] board) {
          for(int i = 0; i < 9; i++)
              for(int j = 0; j < 9; j++) c[i][j] = board[i][j];
          
          solveHelper(0, 0);
          for(int i = 0; i < 9; i++){
              for(int j = 0; j < 9; j++) board[i][j] = c[i][j];
          }
      }
  };
  