feat: add Sudoku Solver LeetCode (#1162)

Co-authored-by: David Leal <halfpacho@gmail.com>

leetcode/DIRECTORY.md

@@ -26,6 +26,7 @@
 | 29   | [Divide Two Integers](https://leetcode.com/problems/divide-two-integers/)                                                       | [C](./src/29.c)   | Medium     |
 | 32   | [Longest Valid Parentheses](https://leetcode.com/problems/longest-valid-parentheses/)                                           | [C](./src/32.c)   | Hard       |
 | 35   | [Search Insert Position](https://leetcode.com/problems/search-insert-position/)                                                 | [C](./src/35.c)   | Easy       |
+| 37   | [Sudoku Solver](https://leetcode.com/problems/sudoku-solver/)                                                                   | [C](./src/37.c)   | Hard       |
 | 38   | [Count and Say](https://leetcode.com/problems/count-and-say/)                                                                   | [C](./src/38.c)   | Easy       |
 | 42   | [Trapping Rain Water](https://leetcode.com/problems/trapping-rain-water/)                                                       | [C](./src/42.c)   | Hard       |
 | 53   | [Maximum Subarray](https://leetcode.com/problems/maximum-subarray/)                                                             | [C](./src/53.c)   | Easy       |

leetcode/src/37.c

@@ -0,0 +1,88 @@
+int** initSet(int size){
+    int** result = (int**) malloc(size * sizeof(int*));
+    for (int i = 0; i < size; i++) {
+        result[i] = (int*)calloc(size, sizeof(int));
+    }
+    
+    return result;    
+}
+
+// Returns the id of triplet which the point (i, j) belongs to
+int getTripletId(int i, int j){
+    return (i / 3) * 3 + (j / 3);
+}
+
+// Recursive function which populates sudoku board.
+bool sudokuSolver(int startI, int startJ, char** board, int boardSize, int* boardColSize, int** horizontalsSets, int** verticalsSets, int** tripletsSets){
+    for (int i = startI; i < boardSize; i++) {
+        for (int j = startJ; j < boardColSize[i]; j++) {
+            if (board[i][j] != '.'){
+                continue;
+            }
+            
+            // Find the sets of the current point (i, j)
+            int* horizontalSet = horizontalsSets[i];
+            int* verticalSet = verticalsSets[j];
+            int* tripletsSet = tripletsSets[getTripletId(i, j)];
+            
+            for (int z = 1; z < 10; z++) {
+                if (horizontalSet[z] || verticalSet[z] || tripletsSet[z]){
+                    continue;
+                }
+                
+                // If the z doesn't belong to occupations sets, we check this value to be in place
+                horizontalSet[z] = 1;
+                verticalSet[z] = 1;
+                tripletsSet[z] = 1;
+                
+                if (sudokuSolver(i, j + 1, board, boardSize, boardColSize, horizontalsSets, verticalsSets, tripletsSets)){
+                    board[i][j] = z + '0';
+                    return true;
+                }
+                
+                horizontalSet[z] = 0;
+                verticalSet[z] = 0;
+                tripletsSet[z] = 0;
+            }
+            
+            // We tried all possible values in range 1-10. No variants - returns false;
+            return false;
+        }
+
+        // startJ to begin of the row.
+        startJ = 0;
+    }
+    
+    // Reach it when the end of the board - then all previous values are setup correctly.
+    return true;
+}
+
+// Use backtracking
+void solveSudoku(char** board, int boardSize, int* boardColSize){
+    // Declare sets for cheking occupation of numbers by horizontals, verticals lines and triplets.
+    int** horizontalsSets = initSet(boardSize + 1);
+    int** verticalsSets = initSet(boardSize + 1);
+    int** tripletsSets = initSet(getTripletId(boardSize + 1, boardSize + 1));
+
+    // Populate sets with values from the board.
+    for (int i = 0; i < boardSize; i++) {
+        for (int j = 0; j < boardColSize[i]; j++) {
+            if (board[i][j] == '.'){
+                continue;
+            }
+            
+            int value = board[i][j] - '0';
+            horizontalsSets[i][value] = 1;
+            verticalsSets[j][value] = 1;
+            tripletsSets[getTripletId(i, j)][value] = 1;
+        }
+    }
+
+    // Solving
+    sudokuSolver(0, 0, board, boardSize, boardColSize, horizontalsSets, verticalsSets, tripletsSets);
+
+    // Free resources
+    free(horizontalsSets);
+    free(verticalsSets);
+    free(tripletsSets);
+}