36. Valid Sudoku.cpp

Determine if a 9 x 9 Sudoku board is valid. Only the filled cells 
need to be validated according to the following rules:

Each row must contain the digits 1-9 without repetition.
Each column must contain the digits 1-9 without repetition.
Each of the nine 3 x 3 sub-boxes of the grid must contain the digits 1-9 without repetition.
Note:

A Sudoku board (partially filled) could be valid but is not necessarily solvable.
Only the filled cells need to be validated according to the mentioned rules.
 

Example 1:


Input: board = 
[["5","3",".",".","7",".",".",".","."]
,["6",".",".","1","9","5",".",".","."]
,[".","9","8",".",".",".",".","6","."]
,["8",".",".",".","6",".",".",".","3"]
,["4",".",".","8",".","3",".",".","1"]
,["7",".",".",".","2",".",".",".","6"]
,[".","6",".",".",".",".","2","8","."]
,[".",".",".","4","1","9",".",".","5"]
,[".",".",".",".","8",".",".","7","9"]]
Output: true
Example 2:

Input: board = 
[["8","3",".",".","7",".",".",".","."]
,["6",".",".","1","9","5",".",".","."]
,[".","9","8",".",".",".",".","6","."]
,["8",".",".",".","6",".",".",".","3"]
,["4",".",".","8",".","3",".",".","1"]
,["7",".",".",".","2",".",".",".","6"]
,[".","6",".",".",".",".","2","8","."]
,[".",".",".","4","1","9",".",".","5"]
,[".",".",".",".","8",".",".","7","9"]]
Output: false
Explanation: Same as Example 1, except with the 5 in the top left corner being 
modified to 8. Since there are two 8's in the top left 3x3 sub-box, it is invalid.
 

Constraints:

board.length == 9
board[i].length == 9
board[i][j] is a digit or '.'.







class Solution {
public:
    
    bool checkRow(vector<vector<char>>& board) {
        for (int j = 0; j < 9; j++) {
            vector <int> freq(10, 0);
            for (int i = 0; i < 9; i++) {
                if (board[i][j] == '.') continue;
                freq[board[i][j] - '0']++;
            }
            for (auto &x: freq) {
                if (x > 1) return false; // multiple same elements in one column
            }
        }
        return true;
    }
    
    bool checkCol(vector<vector<char>>& board) {
        for (int i = 0; i < 9; i++) {
            vector <int> freq(10, 0);
            for (int j = 0; j < 9; j++) {
                if (board[i][j] == '.') continue;
                freq[board[i][j] - '0']++;
            }
            for (auto &x: freq) {
                if (x > 1) return false; // multiple same elements in one row
            }
        }
        return true; 
    }
    
    bool checkSubBox(vector<vector<char>>& board) {
        for (int i = 0; i < 9; i += 3) {
            for (int j = 0; j < 9; j += 3) {
                vector <int> freq(10, 0);
                for (int p = i; p < i + 3; p++){
                    for (int q = j; q < j + 3; q++) {
                        if (board[p][q] == '.') continue;
                        freq[board[p][q] - '0']++;
                    }
                }
                for (auto &x: freq) {
                    if (x > 1) return false; // multiple same elements in one sub box
                }
            }
        }
        return true;
    }
    
    bool isValidSudoku(vector<vector<char>>& board) {
        return checkRow(board) && checkCol(board) && checkSubBox(board);
    }
};