Write a program to solve a Sudoku puzzle by filling the empty cells.
Empty cells are indicated by the character '.'
.
You may assume that there will be only one unique solution.
A sudoku puzzle...
...and its solution numbers marked in red.
题目的输入保证是有效的。用回溯法我们只要检查新加入的值能否在行、列以及小方块里有效即可,没有必要检查整个矩阵。
其实就是对于每一个'.' 尝试1~9 9个值,如果合法,就继续查下一个'.' ,如果非法,尝试下一个值。
若对于某一个'.' 1~9都尝试过且都失败了,就是无解,可以提前返回。
此题是边检查边求解。。
1 class Solution { 2 public: 3 bool isValidSudoku(vector<vector<char> > &board, int x, int y) { 4 int row, col; 5 6 // Same value in the same column? 7 for (row = 0; row < 9; ++row) { 8 if ((x != row) && (board[row][y] == board[x][y])) { 9 return false; 10 } 11 } 12 13 // Same value in the same row? 14 for (col = 0; col < 9; ++col) { 15 if ((y != col) && (board[x][col] == board[x][y])) { 16 return false; 17 } 18 } 19 20 // Same value in the 3 * 3 block it belong to? 21 for (row = (x / 3) * 3; row < (x / 3 + 1) * 3; ++row) { 22 for (col = (y / 3) * 3; col < (y / 3 + 1) * 3; ++col) { 23 if ((x != row) && (y != col) && (board[row][col] == board[x][y])) { 24 return false; 25 } 26 } 27 } 28 29 return true; 30 } 31 32 // 内部check,能否找到结果用bool返回值标示 33 bool internalSolveSudokuCheck(vector<vector<char> > &board) { 34 for (int row = 0; row < 9; ++row) { 35 for (int col = 0; col < 9; ++col) { 36 if ('.' == board[row][col]) { 37 // 对每一个'.' 尝试 1~9,如果不合法,立刻返回 38 for (int i = 1; i <= 9; ++i) { 39 board[row][col] = '0' + i; 40 41 if (isValidSudoku(board, row, col)) { 42 if (internalSolveSudokuCheck(board)) { 43 return true; 44 } 45 } 46 //如果不合法,回复'.',进入下次尝试 47 board[row][col] = '.'; 48 } 49 //对于某一个'.',尝试1~9都不行,就返回false 50 return false; 51 } 52 } 53 } 54 55 return true; 56 } 57 58 void solveSudoku(vector<vector<char> > &board) { 59 internalSolveSudokuCheck(board); 60 } 61 };