**Introduction**

In this article, we will discuss the Valid Sudoku problem. It is an easy question that has been asked in a few competitive coding platforms. Throughout the article, we will discuss the problem statement and some examples and explanations followed by a solution code.

This is a two-part article. This is the second part in which we shall discuss the second variation of the Sudoku Solver problem. The first part of the article can be found __here__**.**

Sudoku is a 9x9 puzzle where the board is divided into nine rows and nine columns. It is further divided into nine small grids of 3x3 size each. An element in sudoku is present exclusively in a row or a column, or a grid.

Recommended topic about, __kth largest element in an array__, and __Euclid GCD Algorithm__

**Problem Statement**

Given a 9x9 sudoku board. Determine whether the given sudoku board is valid or not. Only the filled cells need to be validated according to the following constraints:

- Each row must contain the digits 1-9 without repetition.
- Each column must contain the digits 1-9 with repetition.
- Each of the 3x3 sub-boxes of the grid must contain the digits 1-9 without repetition.

- It should be noted that a Sudoku board (filled partially or fully) may or may not be solvable.
- Check only the cells which are filled. Empty cells are to be ignored.
- Each input will be a 2D char matrix. It will contain numbers and dots (.), where a dot represents a blank cell.

**Example and Explanation**

Let us see some examples to understand better.

Consider an input sequence-

INPUT_1

{'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'}

The sudoku board will look like this-

(source: __Sudoku Solver Visualizer__)

OUTPUT_1

The Sudoku is Valid.

Let us consider another input sequence-

INPUT_2

{'5','3','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'}

This sudoku board will look like-

(source: __Sudoku Solver Visualizer__)

OUTPUT_2

The Sudoku is Invalid.

We are supposed to check whether the given sudoku board is valid or not. A sudoku board is considered valid if it has unique values in each row, column, and grid. In Input_1, all the cells contain unique numbers from 1-9; thus, it is a valid sudoku board. In Input_2, the first row contains element 3 twice (in the same row and same grid), due to which it is invalid sudoku.

**Approach**

Letâ€™s head straight to the approach of the Valid Sudoku problem.

- Since we are dealing with sudoku, it is guaranteed that our input will always be a 2D matrix of 9x9 size.
- In the solution code provided, we will be traversing the input matrix row-wise. We will first check if the element is unique in its row, then the column, then the 3x3 grid.
- For this purpose, we will be maintaining two 2D boolean vectors. These will track that at any given position, the elements do not repeat.
- In the solution code, the two vectors are named
**cols**and**grid**. The two vectors are initialized with false. -
When traversing the sudoku board, three things need to be considered:
**row-wise info: how many rows are to be checked?****column-wise info: how many columns are to be checked?****grid-wise info: how many grids are we supposed to check?**

- If we think about our approach, we are just looking one row at a time. So for that, a 1D vector is sufficient. For column-wise info, we will be traversing all nine columns, but each column has nine rows. So for that, we make the 2D vector
**cols**. Finally, we will be storing the validation check for all nine 3x3 grids. We can use a map for that, but for simplicity, we will again use a 2D vector of size 9x9. - Now we start with the main idea. Start loop
**i**from 0 up to 8. For each column i, create a 1D vector**row**and initialize it with false. Start another loop**j**from 0 up to 8. For each element in the row at position [i][j] check if it is a digit or not. We do this because we are only concerned with the numbers and not the dot (.) that denotes the blank cell. - For each digit in a row, we create a variable
**idx**. For our approach, the boolean value at each index of vector row denotes that element is present or not. For example, at ith index, we will store if the digit i is present in that row or not. - We then perform three checks to check if the element idx is already present in our sudoku or not.
- We first check if idx is already present in that row or not. If present, we return false and exit the function. If not present, we set the idx
^{th}element in the**row**vector as true. - Then, we check if idx is present in the column. To do so, we check the value at cols[j][idx]. If it is true, we return false and exit our function. Else, we update it to true.
- Finally, we check if idx is present in the grid or not. For that, we find its grid index and store it in the variable
**gridIdx**. To find the grid, we use the formula ((i/3)*3 + (j/3)). If the value at grid[gridInx][idx] is true, we return false and exit the function. Else, we update it to true. - After both our loops exhaust, we return true, which denotes that our sudoku board is correct and contains unique values at the required positions.

Before moving to the solution code, the readers can write pseudocode to understand the problem better.

*The readers can also practice the Valid Sudoku problem on our platform *__Coding Ninjas Studio__* by clicking here: *__Sudoku?__* *