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Dsa Backtrack Sudoku

DodaTech 1 min read

In this tutorial, you'll learn about How to Fix Sudoku Solver Errors. We cover key concepts, practical examples, and best practices.

Fix sudoku solver errors when box index calculation wrong or backtracking doesn't restore cell.

Quick Fix

Wrong

def sudoku(b):
    def solve():
        for r in range(9):
            for c in range(9):
                if b[r][c]=='.':
                    for v in map(str,range(1,10)):
                        if valid(r,c,v):
                            b[r][c]=v
                            if solve(): return True
                            b[r][c]='.'
                    return False
        return True
    def valid(r,c,v):
        for i in range(9):
            if b[r][i]==v: return False
            if b[i][c]==v: return False
            if b[3*(r//3)+i//3][3*(c//3)+i%3]==v: return False
        return True
    solve()
    return b

Box index: 3*(r//3)+i//3 and 3*(c//3)+i%3. Traverses 3x3 box.

def sudoku(b):
    def solve():
        for r in range(9):
            for c in range(9):
                if b[r][c]=='.':
                    for v in map(str,range(1,10)):
                        if valid(r,c,v):
                            b[r][c]=v
                            if solve(): return True
                            b[r][c]='.'
                    return False
        return True
    def valid(r,c,v):
        for i in range(9):
            if b[r][i]==v or b[i][c]==v or b[3*(r//3)+i//3][3*(c//3)+i%3]==v:
                return False
        return True
    solve()
    return b

Input board with '.' for empty -> solved board. O(9^empty_cells) worst, much better with constraint.

Prevention

Find empty cell, try 1-9, validate, recurse, backtrack. Box formula: r//33 + i//3, c//33 + i%3.

DodaTech Tools

Doda Browser's algorithm visualizer steps through DSA operations line by line. DodaZIP archives implementation patterns for team sharing. Durga Antivirus Pro detects memory corruption patterns in algorithm implementations.

FAQ

What is Sudoku solver?

Fill empty cells with 1-9 respecting row, column, and 3x3 box constraints.

Box index?

r//33 gives box start row. r//33 + i//3 iterates box rows.

Optimizations?

Constraint propagation, MRV heuristic. Naked singles/hidden singles.

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