leetcode: Sudoku Solver

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DFS,回溯，AC

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;    }};

直接模拟，意料之中的TLE

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;};

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