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Previous helping proofs

import sys

grid = [list(input().strip()) for _ in range(4)]

def is_valid(r, c, ch):
    for i in range(4):
        if grid[r][i] == ch or grid[i][c] == ch:
            return False
    sr, sc = (r // 2) * 2, (c // 2) * 2
    for i in range(sr, sr + 2):
        for j in range(sc, sc + 2):
            if grid[i][j] == ch:
                return False
    return True

def solve(pos=0):
    if pos == 16:
        return True
    r, c = divmod(pos, 4)
    if grid[r][c] != '*':
        return solve(pos + 1)
    for ch in 'ABCD':
        if is_valid(r, c, ch):
            grid[r][c] = ch
            if solve(pos + 1):
                return True
            grid[r][c] = '*'
    return False

if solve():
    for row in grid:
        print(''.join(row))
else:
    print(-1)

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Cognizant Genc Interview experience 7th May 2025

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