37 sudoku

+/**
+ * @param {character[][]} board
+ * @return {void} Do not return anything, modify board in-place instead.
+ */
+var solveSudoku = function(board) {
+    let empty = {};
+    let rows = new Array(9).fill(void 0).map(()=>new Set());
+    let cols = new Array(9).fill(void 0).map(()=>new Set());
+    let blocks = new Array(9).fill(void 0).map(()=>new Set());
+
+    for (let i=0;i<9;i++) {
+        for (let j = 0; j < 9; j++) {
+            let block = block_no(i, j);
+            let v = board[i][j];
+            if (v !== '.') {
+                rows[i].add(v);
+                cols[j].add(v);
+                blocks[block].add(v);
+                mark({block, i, j}, v);
+            } else {
+                mark({block, i, j}, v);
+            }
+        }
+    }
+
+    function block_no(i, j) {
+        return (~~(i/3))*3 + ~~(j/3);
+    }
+    function mark({block, i, j}, v) {
+        for (let n=0;n<9;n++){
+            // not empty cell
+            if ((i === n || j === n) && v !== '.') {
+                if (empty[i+','+j]) {
+                    delete empty[i+','+j];
+                }
+                continue;
+            }
+
+            // cols
+            if (board[i][n] === '.') {
+                let e = empty[i+','+n] = empty[i+','+n] || {block: block_no(i, n), i, j:n, maybe: new Set(['1','2','3','4','5','6','7','8','9'])};
+                e.maybe.delete(v);
+                if (!e.maybe.size) {
+                    delete empty[i+','+n];
+                }
+            }
+            
+            // rows
+            if (board[n][j] === '.') {
+                let e = empty[n+','+j] = empty[n+','+j] || {block: block_no(n, j), i:n, j, maybe: new Set(['1','2','3','4','5','6','7','8','9'])};
+                e.maybe.delete(v);
+                if (!e.maybe.size) {
+                    delete empty[n+','+j];
+                }
+            }
+            
+            // blocks
+            let bi = ~~(block/3);
+            let bj = block % 3;
+            for (let bn=0;bn<9;bn++) {
+                let i = bi*3 + ~~(bn/3);
+                let j = bj*3 + bn % 3;
+                if (board[i][j] === '.') {
+                    let e = empty[i+','+j] = empty[i+','+j] || {block: block_no(i, j), i, j, maybe: new Set(['1','2','3','4','5','6','7','8','9'])};
+                    e.maybe.delete(v);
+                    if (!e.maybe.size) {
+                        delete empty[i+','+j];
+                    }
+                }
+            }
+        }
+    }
+
+    function fill(i, j, block, v) {
+        board[i][j] = v;
+        empty[i+','+j].value = v;
+        rows[i].add(v);
+        cols[j].add(v);
+        blocks[block].add(v);
+    }
+
+    function unfill(i, j, block, v) {
+        board[i][j] = '.';
+        empty[i+','+j].value = void 0;
+        rows[i].delete(v);
+        cols[j].delete(v);
+        blocks[block].delete(v);
+    }
+
+    function deal() {
+        let blanks = Object.keys(empty).map(k=>empty[k]).filter(m=>!m.value).sort((a,b)=>{
+            return a.maybe.size - b.maybe.size;
+        });
+
+        for (let e of blanks) {
+            let {block, i, j, value} = e;
+            if (value) {continue;}
+            let union = new Set([...rows[i], ...cols[j], ...blocks[block]]);
+            let maybe = new Set(['1','2','3','4','5','6','7','8','9'].filter(v=>!union.has(v)));
+            
+            if (!maybe.size) {
+                return false;
+            }
+            for (let v of [...maybe]) {
+                // set for testing
+                if (rows[i].has(v) || cols[j].has(v) || blocks[block].has(v)) {
+                    return false;
+                }
+                fill(i, j, block, v);
+                if (deal()) {
+                    break; // found
+                } else {
+                    // rollback
+                    unfill(i, j, block, v);
+                    continue; // test next
+                }
+            }
+
+            if (board[i][j] === '.') {
+                return false;
+            }
+        }
+
+        return true;
+    }
+
+    deal();
+};
+
+var sudoku = [["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"]];
+solveSudoku(sudoku);
+console.info(sudoku);