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Data Structures & Algorithms June 15, 2020 3 min read

Leetcode 37 Sudoku Solver Java Solution

// SOLVING THIS WITH AN AI ASSISTANT (2026)

If you are working through this problem with an AI coding assistant — Claude, ChatGPT, Cursor chat, Gemini, GitHub Copilot, Aider, or any agent — the goal isn’t to ask for the answer. It is to use the tool to understand the pattern. The prompt sequence I’d run:

  1. Spec it back to me first. “In your own words, what is this problem actually testing? What’s the smallest example that fails the naive approach?”
  2. Brute-force first, optimize after. “Write the simplest correct solution, even if it’s O(n²). Don’t optimize. Just make it correct, with comments explaining each step.”
  3. Ask for the upgrade. “Now show me the optimal solution. What insight makes it possible? What pattern is this an instance of?”
  4. Stress-test it. “Generate 10 edge cases — empty input, single element, duplicates, max size, sorted, reverse-sorted. Run my solution against each.”

The pattern matters more than the answer. If the agent just hands you optimized code, you’ve trained yourself to lose interviews.

Write a program to solve a Sudoku puzzle by filling the empty cells.

A sudoku solution must satisfy all of the following rules:

  1. Each of the digits 1-9 must occur exactly once in each row.
  2. Each of the digits 1-9 must occur exactly once in each column.
  3. Each of the the digits 1-9 must occur exactly once in each of the 9 3x3 sub-boxes of the grid.

Empty cells are indicated by the character '.'.


A sudoku puzzle…


…and its solution numbers marked in red.

Note:

Solution 1:

class Solution {
    public void solveSudoku(char[][] board) {
        helper(board);
    }
    
    public boolean helper(char[][] board){
        for(int i=0;i<9;i++){
                for(int j=0;j<9;j++){
                    if(board[i][j]!='.'){
                        continue;
                    }
                    for(char k='1';k<='9';k++){
                        if(isValid(board, i, j, k)){
                            board[i][j] = k;
                           if (helper(board)){
                               return true;
                           }
                            board[i][j] = '.';
                        }
                        
                    }
                    return false;
                }
        }
        return true;
    }
    
    
    public boolean isValid(char[][] board, int row, int column, char k){
            for(int i=0;i<9;i++){
                if(board[row][i]!='.' && board[row][i]==k) return false;
                
                if(board[i][column]!='.' && board[i][column]==k) return false;
                
                if(board[3 * (row / 3) + i / 3][3 * (column / 3) + i % 3]!='.' && board[3 * (row / 3) + i / 3][3 * (column / 3) + i % 3]== k) return false;
            }
        
        return true;
            
    }
}

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