leetcode 37. Sudoku Solver

1.题目

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...

求解数独。。。

2.思路 

利用递归，先放一个数在格子上，如果合格，再放下一个格子，如果不合格，回溯。

class Solution {public:    bool isValid(vector<vector<char>>& board,int position) { //测试当前position位置上的数是否合法    int row = position/9;    int col = position%9;    int gid = (row/3)*3 + col/3; //第(row/3*3 + col/3)个小方块的起始位置    for(int i=0;i<9;i++)    {        //check row        if(i != col && board[row][i] == board[row][col])            return false;                //check col        if(i != row && board[i][col] == board[row][col])            return false;                //check subgrid        int r = gid/3*3+i/3;        int c = gid%3*3+i%3;        if((r != row || c != col) && board[r][c] == board[row][col])            return false;    }    return true;}bool solve(vector<vector<char>>& board,int position){    if(position == 81)        return true;        int row = position/9;    int col = position%9;        if(board[row][col] == '.')    {        for(int k=1;k<=9;k++)        {            board[row][col] = k + '0';            if(isValid(board,position))                if(solve(board,position+1))                    return true;        }        board[row][col] = '.';        return false;    }    else return  solve(board,position+1);}void solveSudoku(vector<vector<char>>& board) {    solve(board,0);}};