# [Leetcode] Sudoku Solver

Source: Internet
Author: User

Write a program to solve a Sudoku puzzle by filling the empty cells.

Empty cells are indicated by the character`‘.‘`.

You may assume that there will be only one unique solution.

A Sudoku puzzle...

... And its solution numbers marked in red.

The sudoku solver was finally reached. With the basis of valid Sudoku, I learned that using a hash table can easily identify conflicts. The idea is to traverse the Board. This time, we found a blank location and then filtered the possible result set: we removed the numbers in rows, columns, and 3x3, the rest is the possible result set. Then, extract the elements in the result set one by one for in-depth search until the recursive exit is met: ROW = 8 & Col = 8 indicates that the Board traversal is complete and the result is obtained; if the final result returned by the function does not meet that condition, it indicates that there is no solution.

The main problem I encountered in solving this problem was the ambiguity of the recursive exit judgment, and I always liked to upload a reference when I passed the board, which caused a deep search to change the board .. This is a bug, which was also encountered in Word breakii. The overall understanding of this question still needs to be further elaborated.

` 1 void solver(vector<vector<char>> board, int row, int col, vector<vector<char>> &ret) { 2     if (board[row][col] != ‘.‘) { 3         if (row == 8 && col == 8) { 4             ret = board; 5             return; 6         } 7         else if (++col >= 9) { 8             ++row; 9             col = 0;10         }11         solver(board, row, col, ret);12         return;13     }14     15     // calculate solve space16     unordered_set<char> set = preset;17     // filter colum18     for (int k = 0; k < 9; k++) {19         char searched = board[k][col];20         if (searched != ‘.‘ && set.find(searched) != set.end()) {21             set.erase(searched);22         }23     }24     // filter row25     for (int k = 0; k < 9; k++) {26         char searched = board[row][k];27         if (searched != ‘.‘ && set.find(searched) != set.end()) {28             set.erase(searched);29         }30     }31     // filter 3*332     int square_row_offset = row / 3;33     int square_col_offset = col / 3;34     for (int k = square_row_offset * 3; k < square_row_offset * 3 + 3; k++) {35         for (int l = square_col_offset * 3; l < square_col_offset * 3 + 3; l++) {36             char searched = board[k][l];37             if (searched != ‘.‘ && set.find(searched) != set.end()) {38                 set.erase(searched);39             }40         }41     }42     43     for (char ele: set) {44         board[row][col] = ele;45         46         if (row == 8 && col == 8) {47             ret = board;48             return;49         }50         51         solver(board, row, col+1, ret);52     }53 }54 55 56 vector<vector<char>> sudokuSolver(vector<vector<char>> &board) {57     vector<vector<char>> ret;58     solver(board, 0, 0, ret);59     return ret;60 }`

[Leetcode] Sudoku Solver

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