LeetCode 37. Sudoku Solver

class Solution {
public:
    vector<vector<bool>> rowhash; // rowhash[row][1~9] - if the number has been taken in row
    vector<vector<bool>> colhash; // colhash[col][1~9] - if the number has been taken in col
    vector<vector<bool>> boxhash; // box are indexed by 0~8, from left to right, top to bottom
    
    void solveSudoku(vector<vector<char>>& board) {
        int m=board.size(), n=m==0?0:board[0].size();
        if (m==0 || n==0) return;
        
        rowhash.resize(9,vector<bool>(10,false));
        colhash.resize(9,vector<bool>(10,false));
        boxhash.resize(9,vector<bool>(10,false));
        
        for (int i=0;i<m;++i){
            for (int j=0;j<n;++j){
                if (board[i][j]==‘.‘) continue;
                int num=board[i][j]-‘0‘;
                rowhash[i][num] = true;
                colhash[j][num] = true;
                boxhash[i/3*3+j/3][num] = true; // i/3, j/3
            }
        }
        backtrack(board,0,0);
    }
    
    bool backtrack(vector<vector<char>> &board, int i, int j){
        if (i==9) return true;
        if (board[i][j]!=‘.‘){
            auto [next_i,next_j] = nextPos(i,j);
            return backtrack(board,next_i,next_j);
        }else{
            // select a number to fill in the blank
            for (int k=1;k<=9;++k){
                if (!rowhash[i][k] && !colhash[j][k] && !boxhash[i/3*3+j/3][k]){
                    board[i][j] = k+‘0‘;
                    rowhash[i][k] = colhash[j][k] = boxhash[i/3*3+j/3][k] = true;
                    auto [next_i,next_j] = nextPos(i,j);
                    if (backtrack(board,next_i,next_j))
                        return true;
                    board[i][j] = ‘.‘;
                    rowhash[i][k] = colhash[j][k] = boxhash[i/3*3+j/3][k] = false;
                }
            }
        }
        return false;
    }
    
    pair<int,int> nextPos(int i, int j){
        if (j==8) return {i+1,0};
        else return {i,j+1};
    }
};