February 2020 - Solution

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February 2020 Challenge

We received several solutions.

Harald Bögeholz found a solution with only snakes: [[22, 2], [26, 8], [24, 21], [40, 25], [51, 38], [57, 42], [78, 53], [76, 61], [86, 74], [99, 82]]
Latchezar Christov also sent us a solution with only three snakes/ladders: [[35,15],[67,61],[99,9]]
The best solution we received is from Asaf Zimmerman: [[61,56],[60,54],[59,53],[58,54],[57,55],[14,38],[35,13],[92,66],[97,89],[84,64]], which is accurate for 15 digits after the decimal point.

Peter Huggins has an efficient way to halve the search space:
"The solution was found by considering the board as a left half and right half. At the end of the left half, we prescribed five consecutive snakes with variable left endpoints, so that the only way to make it to the right half of the board was to land on the square immediate after the "blockade" of five snakes. This allowed computing expected number of moves completely separate for left and right half and then adding the two. A search space of such restricted choices of snakes/ladders in the left half was evaluated, and similarly the right half. Then the sum of left and right expectation that was closest to the target sum was found using the standard double linear scan approach after sorting."

The solution we intended was [[80, 31], [79, 41], [78, 59], [68, 26], [69, 53], [82, 58], [84, 97], [72, 93], [73, 23], [83, 84]]
which is the ASCII values of "ponderthis" as the first number and the decimal expansion of Pi as the second.

This gives an expected time of exactly
12069717322599589450686796528518338401917187078111730231579496649/180202309394278154198991500658119021487803384043920462457077760, which is approximately 66.978705007555420778...