leetcode: Sudoku Solver

class Solution {
public:
void solveSudoku(vector<vector<char> > &board) {
solve( board);
}
bool isvalid( vector< vector< char> > &board, int row, int col){
for( int i = 0; i < board.size(); ++i){
if( i != row && board[i][col] == board[row][col])
return false;
}
for( int j = 0; j < board[0].size(); ++j){
if( j != col && board[row][j] == board[row][col])
return false;
}
int blocki = row / 3;
int blockj = col / 3;
for( int i = blocki * 3; i < blocki * 3 + 3; ++i){
for( int j = blockj * 3; j < blockj * 3 + 3; ++j){
if( i != row && j != col && board[i][j] == board[row][col])
return false;
}
}
return true;
}
bool solve( vector< vector< char> > &board){
for( int i = 0; i < board.size(); ++i){
for( int j = 0; j < board[i].size(); ++j){
if( board[i][j] == '.'){
for( int k = '1'; k <= '9'; ++k){
board[i][j] = k;
if( isvalid( board, i, j) && solve( board))
return true;
board[i][j] = '.';
}
return false;
}
}
}
return true;
}
};

class Solution {
public:
void solveSudoku(vector<vector<char> > &board) {
generateCandidates( board, candidates);
while( true){
for( int i = 0; i < board.size(); ++i){
for( int j = 0; j < board[i].size(); ++j){
if( candidates[i][j].size() == 1){
board[i][j] = *(candidates[i][j].begin());
if(solved( board))
return;
candidates[i][j].erase( candidates[i][j].begin());
generateCandidates( board, candidates);
}
}
}
}
}
bool solved( vector< vector< char> > &board){
for( int i = 0; i < board.size(); ++i)
for( int j = 0; j < board[i].size(); ++j)
if( board[i][j] == '.')
return false;
return true;
}
void generateCandidates( vector< vector< char> > &board, vector< vector< set< char> > > &candidates){
set< char> nums;
for( int i = '1'; i <= '9'; ++i)
nums.insert(i);
for( int i = 0; i < 9; ++i){
candidates.push_back( vector< set< char> >( 9, nums));
}
for( int i = 0; i < board.size(); ++i){
for( int j = 0; j < board[i].size(); ++j){
if( board[i][j] == '.'){
modifyCandidates( board, candidates, i, j);
}
}
}
}
void modifyCandidates( vector< vector< char> > &board, vector< vector< set< char> > > &candidates, int row, int col){
for( int i = 0; i < board.size(); ++i){
if( board[i][col] != '.'){
if( candidates[row][col].count( board[i][col]))
candidates[row][col].erase( board[i][col]);
}
}
for( int j = 0; j < board[0].size(); ++j){
if( board[row][j] != '.'){
if( candidates[row][col].count( board[row][j]))
candidates[row][col].erase( board[row][j]);
}
}
int blocki = row / 3;
int blockj = col / 3;
for( int i = blocki * 3; i <= blocki * 3 + 2; ++i){
for( int j = blockj * 3; j <= blockj * 3 + 2; ++j){
if( board[i][j] != '.'){
if( candidates[row][col].count( board[i][j]))
candidates[row][col].erase( board[i][j]);
}
}
}
}
private:
vector< vector< set< char> > > candidates;
};