Mock interview: Jump to last stone

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	using System.Collection.Generic;
	public class Solution {
	public static void Main()
	{
	var result = CanCross(new int[]{0,1,3,5,6,8,12,17});
	Console.WriteLine("testing..");
	}
	//[0,1,3,5,6,8,12,17]
	public static bool CanCross(int[] stones) {
	if(stones == null || stones.Length == 0)
	return false;
	var length = stones.Length; //8
	// var dp = new List<int>[length];
	var dp = new Dictionary<int, List<int>>();
	for(int i = 0; i < length; i++)
	{
	// dp[i] = new List<int>(); //
	dp[i] = new Dictionary<int, List<int>>();
	}
	// dynamic programming from bottom up
	// [0,1,3,5,6,8,12,17]
	// dp[0].Add(0); // [0]
	// // dp[17]
	// {
	// 0 , 1, 2, 3 .. length
	// }
	// hash table?
	// {
	// 0 : list
	// 1 : list
	// 3 : list
	// ...
	// 17: list.length > 0
	// }
	var hashset = new HashSet<int>(stones);
	// [0, 1, 3, ..., 17
	for(int i = 0; i < length; i++) // mix index with stones[index]
	{
	var index = stones[i]; // stone number
	var list = dp[i];
	foreach(var item in list)
	{
	// same, +1, -1 jump
	var sameStep = item;
	var plusOne = item + 1;
	var minusOne = item - 1; // -1 < -
	var choices = new int[]{sameStep, plusOne, minusOne};
	foreach(var item in choices)
	{
	// item are the available k units
	// You want to make sure the item are forward
	if( item > 0)
	{
	var target = index + item;
	if(hashSet.Contains(target))
	{
	if(!dp.ContainsKey(index + item))
	{
	dp.Add(index + item, new List<int>());
	}
	// stone number = not index
	dp[index + item].Add(item);
	}
	}
	}
	}
	}
	}
	return dp[stones[length - 1]].Count > 0;
	}
	}
	// A frog is crossing a river. The river is divided into x units and at each unit there may or may not exist a stone. The frog can jump on a stone, but it must not jump into the water.
	// Given a list of stones' positions (in units) in sorted ascending order, determine if the frog is able to cross the river by landing on the last stone. Initially, the frog is on the first stone and assume the first jump must be 1 unit.\
	// If the frog's last jump was k units, then its next jump must be either k - 1, k, or k + 1 units.
	// Note that the frog can only jump in the forward direction.
	// Note:
	// * The number of stones is ≥ 2 and is < 1,100.
	// * Each stone's position will be a non-negative integer < 231.
	// * The first stone's position is always 0.
	// Example 1:
	// [0,1,3,5,6,8,12,17]
	// 0 -> 1
	// 1 (0, 1, 2)
	// 3(2) (1, 2, 3)
	// 5 6
	// index K
	// dp -> dp[][]
	// unit
	// dp[0][0][]
	// dp[1][1]
	// dp[3][2] -> k = 3 _>get to
	// dp[5][2]
	// dp[6][] from position 3 or 5
	// {
	// 0 : 0
	// 1 : 1
	// 3 : 2
	// 5 : 2
	// 6 : 1 -> 3
	// ...
	// 17: list.length > 0
	// }
	// 1 or 3
	// dp[6][] -> store as a list
	// dp[17] _> 7 previous positions -> dp[17] -> maximum -> minimum
	// There are a total of 8 stones.
	// The first stone at the 0th unit, second stone at the 1st unit,
	// third stone at the 3rd unit, and so on...
	// The last stone at the 17th unit.
	// Return true. The frog can jump to the last stone by jumping
	// 1 unit to the 2nd stone, then 2 units to the 3rd stone, then
	// 2 units to the 4th stone, then 3 units to the 6th stone,
	// 4 units to the 7th stone, and 5 units to the 8th stone.
	// Example 2:
	// [0,1,2,3,4,8,9,11]
	// Return false. There is no way to jump to the last stone as
	// the gap between the 5th and 6th stone is too large.
	// 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
	// X k = 0
	// X k = 1
	// X k = 2

view raw JumpToLastStone.cs hosted with ❤ by GitHub

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Code shared by interviewer

Here is the gist.

	const _ = require('lodash');
	function sayHello() {
	console.log('Hello, World');
	}
	_.times(5, sayHello);
	var canCross = function(stones) {
	let map = new Map();
	for (let stone of stones) {
	map.set(stone, new Set());
	}
	map.get(stones[0]).add(0);
	for (let i = 0; i < stones.length; i++) {
	let steps = map.get(stones[i]).values();
	for (let step of steps) {
	for (let k = step - 1; k <= step + 1; k++) { // O(1
	if (k > 0 && map.has(stones[i] + k)) {
	map.set(stones[i] + k, map.get(stones[i] + k).add(k));
	}
	}
	}
	}
	return map.get(stones[stones.length - 1]).size > 0;
	};

view raw JumpToLastStone.py hosted with ❤ by GitHub