day 21

Author: sachuverma

Leetcode Daily Challenge/August-2021/21. Sudoku Solver.cpp

@@ -0,0 +1,62 @@
+/*
+Sudoku Solver
+=============
+
+Write a program to solve a Sudoku puzzle by filling the empty cells.
+A sudoku solution must satisfy all of the following rules:
+
+Each of the digits 1-9 must occur exactly once in each row.
+Each of the digits 1-9 must occur exactly once in each column.
+Each of the digits 1-9 must occur exactly once in each of the 9 3x3 sub-boxes of the grid.
+The '.' character indicates empty cells.
+
+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: [["5","3","4","6","7","8","9","1","2"],["6","7","2","1","9","5","3","4","8"],["1","9","8","3","4","2","5","6","7"],["8","5","9","7","6","1","4","2","3"],["4","2","6","8","5","3","7","9","1"],["7","1","3","9","2","4","8","5","6"],["9","6","1","5","3","7","2","8","4"],["2","8","7","4","1","9","6","3","5"],["3","4","5","2","8","6","1","7","9"]]
+Explanation: The input board is shown above and the only valid solution is shown below:
+
+Constraints:
+board.length == 9
+board[i].length == 9
+board[i][j] is a digit or '.'.
+It is guaranteed that the input board has only one solution.
+*/
+
+class Solution {
+public:
+    bool valid(vector<vector<char>>& board, int r, int c, char val) {
+        for(int i=0; i<9; ++i) {
+            if(board[r][i] == val || board[i][c] == val) return false;
+        }
+        
+        for(int i=0; i<3; ++i) {
+            for(int j=0; j<3; ++j) {
+                if(board[(r/3)*3 + i][(c/3)*3 + j] == val) return false;
+            }
+        }
+        
+        return true;
+    }
+    
+    bool backtrack(vector<vector<char>>& board, int r, int c){
+        if(r == 9) return true;
+        if(c == 9) return backtrack(board, r+1, 0);
+        
+        if(board[r][c] != '.') return backtrack(board, r, c+1);
+        
+        for(char ch = '1'; ch <= '9'; ++ch) {
+            if(valid(board, r, c, ch)) {
+                board[r][c] = ch;
+                if(backtrack(board, r, c+1)) return true;
+                board[r][c] = '.';
+            }
+        }
+        
+        return false;
+    }
+    
+    void solveSudoku(vector<vector<char>>& board) {
+        backtrack(board, 0, 0);
+    }
+};
+

Leetcode Daily Challenge/August-2021/README.md

@@ -22,3 +22,4 @@
 | 18. | [Decode Ways](https://leetcode.com/explore/challenge/card/august-leetcoding-challenge-2021/615/week-3-august-15th-august-21st/3902/)                             |                   [cpp](./18.%20Decode%20Ways.cpp)                   |
 | 19. | [Maximum Product of Splitted Binary Tree](https://leetcode.com/explore/challenge/card/august-leetcoding-challenge-2021/615/week-3-august-15th-august-21st/3903/) | [cpp](./19.%20Maximum%20Product%20of%20Splitted%20Binary%20Tree.cpp) |
 | 20. | [Valid Sudoku](https://leetcode.com/explore/challenge/card/august-leetcoding-challenge-2021/615/week-3-august-15th-august-21st/3904/)                            |                  [cpp](./20.%20Valid%20Sudoku.cpp)                   |
+| 21. | [Sudoku Solver](https://leetcode.com/explore/challenge/card/august-leetcoding-challenge-2021/615/week-3-august-15th-august-21st/3905/)                            |                  [cpp](./20Sudoku%20Solver.%20.cpp)                   |