# 37. Sudoku Solver

```java
class Solution {
    public void solveSudoku(char[][] board) {
        backTrack(board,0);
    }
    
    // return boolean to 1. reduce time complexity
    // 2.to make backtrack not revert the value in board 
    private boolean backTrack(char[][] board, int i){
        if(i==81) return true;
        
        int row = i/9;
        int col= i%9;
        
        if(board[row][col]!='.') return backTrack(board, i+1);
        
        for(char ch='1'; ch<='9'; ch++){
            if(!isValid(row, col, ch, board)) continue;
            board[row][col]=ch;
            if(backTrack(board, i+1)){
                return true;
            }
            board[row][col]='.';
        }
        return false;
    }
    
    private boolean isValid(int i, int j, char ch, char[][] board){
        //check the 3*3 subbox
        for(int row=i/3*3; row<(i/3+1)*3; row++){
            for(int col=j/3*3; col<(j/3+1)*3; col++){
                if(ch==board[row][col]){
                    return false;
                }
            }
        }
        
        //check the row
        for(int col=0; col<board[0].length; col++){
            if(ch==board[i][col]) return false;
        }
        
        //check the column
        for(int row=0; row<board.length; row++){
            if(ch==board[row][j]) return false;
        }
        return true;
    }
}
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://ucan.gitbook.io/notes/backtrack/37.-sudoku-solver.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.