[Leetcode]Sudoku Solver

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...

class Solution {public:    /*algorithm: DFS    */    void solveSudoku(vector<vector<char> > &board) {        dfs(board, 0, 0);    }    bool dfs(vector<vector<char> > &board, int x, int y) {        if (x == 9) return true;        if (y >= 9) return dfs(board, x + 1, 0);        if (board[x][y] == '.') {            for (int k = 1; k <= 9; ++k) {                board[x][y] = k + '0';                if (isValid(board, x , y)) {                    if (dfs(board, x, y + 1)) return true;                }                board[x][y] = '.';            }        } else {            return dfs(board, x, y + 1);        }        return false;    }    bool isValid(vector<vector<char> > &board, int x, int y) {        char c = board[x][y];        for (int col = 0; col < 9; ++col) {            if (col != y && c == board[x][col]) return false;        }        for (int row = 0; row < 9; ++row) {            if (row != x && c == board[row][y]) return false;        }        int topx = x/3*3,topy=y/3*3;        for (int row = topx; row < topx+3; ++row) {            for (int col = topy; col < topy+3; ++col) {                if ((row != x || col != y) && c == board[row][col]) return false;            }        }        return true;    }};