DIY: Frog Jump

Problem statement

A river is divided into units. At each unit, there may or may not be a stone. A frog has to cross the river. It can jump on a stone, but it must not jump into the water.

You are given a list of the positions of the stones in a sorted and ascending order. You have to determine if the frog can cross the river by landing on the last stone. The frog starts at the first stone. Let’s assume that the first jump is 1 unit. If the frog’s last jump was k units, its next jump must either be k - 1, k, or k + 1 units. The frog can only jump forward.

You will be given an array of integers called stones, which will contain the positions of the stones. You will return true if the frog can cross the river using the last stone. Otherwise, you will return false.