Create Sudoku_solver.js

02. Algorithms/07. Backtracking/Sudoku Solver/Sudoku_solver.js

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+/**
+ * https://leetcode.com/problems/sudoku-solver/
+ * 
+ * Recursion | Backtracking.
+ * n won't exeed 9. So, It's ok
+ * Time O(n^n) | Space O(n)
+ * @param {character[][]} board
+ * @return {void} Do not return anything, modify board in-place instead.
+ */
+var solveSudoku = function(board) {
+
+
+    const ROWS = board.length;
+    const COLS = board[0].length;
+
+    const isValid = (num, row, col) => {
+        // check the row.
+        for(let i = 0; i < 9; i++) {
+            if(board[row][i] === num) return false;
+        }
+
+        // check the col.
+        for(let i = 0; i < 9; i++) {
+            if(board[i][col] === num) return false; 
+        }
+
+        // check the 3*3 grid.
+        const rowStart = Math.floor(row/3) * 3;
+        const colStart = Math.floor(col/3) * 3;
+        for(let i = rowStart; i < rowStart+3; i++) {
+            for(let j = colStart; j < colStart+3; j++) {
+                if(board[i][j] === num) return false;
+            }
+        }
+
+        // we can use the number.
+        return true;
+
+    }
+
+    const dfs = (row,col) => {
+        if(row === ROWS) return true;
+
+        const nextRow = col >= 8 ? row + 1 : row;
+        const nextCol = col >= 8 ? 0 : col + 1;
+
+        if(board[row][col] !== '.') {
+            if(dfs(nextRow, nextCol)) return true;
+        }
+        if(board[row][col] === '.') {
+            for(let i = 1; i < 10; i++) {
+                if(isValid(i.toString(), row, col)) {
+                    board[row][col] = i.toString();
+                    if(dfs(nextRow, nextCol)) return true;
+                    board[row][col] = '.';
+                }
+            }
+        }
+        return false;
+    }
+
+    dfs(0,0)
+};