How to Check Valid Sudoku in C/C++?

Determine if a Sudoku is valid. The Sudoku board could be partially filled, where empty cells are filled with the character ‘.’. A valid Sudoku board (partially filled) is not necessarily solvable. Only the filled cells need to be validated.

sudoku

A valid Sudoku contains three conditions: (1) all rows should contain exactly 1 to 9. (2) all columns should contain exactly 1 to 9. (3) all sub grids (9 of them) should contain exactly 1 to 9.

The best data structure we can use is the STL:set, we need to clean the set before next validation (row, column or grid). To solve a Sudoku, we can use DFS algorithm: Depth First Search (Backtracking) Algorithm to Solve a Sudoku Game

// https://helloacm.com/how-to-check-valid-sudoku-in-cc/
class Solution {
public:
    bool isValidSudoku(vector<vector<char>>& board) {
        unordered_set<int> has;
        for (int i = 0; i < 9; i ++) {
            has.clear();
            // rows
            for (int j = 0; j < 9; j ++) {
                if (board[i][j] != '.') {
                    if (has.count(board[i][j])) {
                        return false;
                    }
                    has.insert(board[i][j]);
                }
            }
            has.clear();
            // columns
            for (int j = 0; j < 9; j ++) {
                if (board[j][i] != '.') {
                    if (has.count(board[j][i])) {
                        return false;
                    }
                    has.insert(board[j][i]);
                }
            }
        }
        for (int i = 0; i < 3; i ++) {
            for (int j = 0; j < 3; j ++) {
                // sub grids
                has.clear();
                for (int x = i * 3; x < i * 3 + 3; x ++) {
                    for (int y = j * 3; y < j * 3 + 3; y ++) {
                        if (board[x][y] != '.') {
                            if (has.count(board[x][y])) {
                                return false;
                            }
                            has.insert(board[x][y]);
                        }
                    }
                }
            }
        }
        return true;
    }
};

See also: Just Another Sudoku Solver in Depth First Search (and BackTracking) Algorithm

Checking a Sudoku to see if it is valid or not in Python with 27 Hash Sets: Teaching Kids Programming – Sudoku Validator/Algorithm using 27 Hash Sets

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