Given two integers n and m, where n represent some paintings numbered from 1 to n and m represent some colours 1 to m with unlimited amount. The task is to find the number of ways to paint the paintings such that no two consecutive paintings have the same colors.
Note: Answer must be calculated in modulo 10^9 +7 as answer can be very large.
Input: n = 4, m = 2 Output: 2 Input: n = 4, m = 6 Output: 750
Asked in : National Instruments
The total number of given color is m and the total paintings are from 1 to n. As per the condition of no two adjacent painting having the same color, first painting can be painted by anyone out of n colors and the rest of any painting can be painted by any of n-1 color except the color used for the painting just preceding that. Hence if we derive the solution for total number of ways,
n * (m-1)^(n-1) is the actual answer.
Now, this can be either calculated by simple iteration or by the method of efficient power calculation in O(logn) time.
Below is the implementation of the above approach:
// If y is odd, multiply
// x with result
if ($y & 1)
$res = ($res * $x) % $p;
// y must be even now
// y = $y/2
$y = $y >> 1;
$x = ($x * $x) % $p;
// Function to calculate the number of ways
function ways($n, $m)
// Answer must be modulo of 10^9 + 7
return (power($m – 1, $n – 1,
$modd) * $m ) % $modd;
// Driver code
$n = 5;
$m = 5;
echo ways($n, $m);
// This code is contributed
// by Arnab Kundu
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