Interview puzzle: Jump Game

问题

Jump Game: Given an array, start from the first element and reach the last by jumping. The jump length can be at most the value at the current position in the array. The optimum result is when you reach the goal in minimum number of jumps.

What is an algorithm for finding the optimum result?

An example: given array A = {2,3,1,1,4} the possible ways to reach the end (index list) are

0,2,3,4 (jump 2 to index 2, then jump 1 to index 3 then 1 to index 4)
0,1,4 (jump 1 to index 1, then jump 3 to index 4)

Since second solution has only 2 jumps it is the optimum result.

回答1:

Overview

Given your array a and the index of your current position i, repeat the following until you reach the last element.

Consider all candidate "jump-to elements" in a[i+1] to a[a[i] + i]. For each such element at index e, calculate v = a[e] + e. If one of the elements is the last element, jump to the last element. Otherwise, jump to the element with the maximal v.

More simply put, of the elements within reach, look for the one that will get you furthest on the next jump. We know this selection, x, is the right one because compared to every other element y you can jump to, the elements reachable from y are a subset of the elements reachable from x (except for elements from a backward jump, which are obviously bad choices).

This algorithm runs in O(n) because each element need be considered only once (elements that would be considered a second time can be skipped).

Example

Consider the array of values a, indicies, i, and sums of index and value v.

i ->  0   1   2   3   4   5   6   7   8   9  10  11  12
a -> [4, 11,  1,  1,  1,  1,  1,  1,  1,  1,  1,  1,  1]
v ->  4  12   3   4   5   6   7   8   9  10  11  12  13

Start at index 0 and consider the next 4 elements. Find the one with maximal v. That element is at index 1, so jump to 1. Now consider the next 11 elements. The goal is within reach, so jump to the goal.

Demo

See here or here with code.

回答2:

Dynamic programming.

Imagine you have an array B where B[i] shows the minimum number of step needed to reach index i in your array A. Your answer of course is in B[n], given A has n elements and indices start from 1. Assume C[i]=j means the you jumped from index j to index i (this is to recover the path taken later)

So, the algorithm is the following:

set B[i] to infinity for all i
B[1] = 0;                    <-- zero steps to reach B[1]
for i = 1 to n-1             <-- Each step updates possible jumps from A[i]
    for j = 1 to A[i]        <-- Possible jump sizes are 1, 2, ..., A[i]
        if i+j > n           <-- Array boundary check
            break
        if B[i+j] > B[i]+1   <-- If this path to B[i+j] was shorter than previous
            B[i+j] = B[i]+1  <-- Keep the shortest path value
            C[i+j] = i       <-- Keep the path itself

The number of jumps needed is B[n]. The path that needs to be taken is:

1 -> C[1] -> C[C[1]] -> C[C[C[1]]] -> ... -> n

Which can be restored by a simple loop.

The algorithm is of O(min(k,n)*n) time complexity and O(n) space complexity. n is the number of elements in A and k is the maximum value inside the array.

Note

I am keeping this answer, but cheeken's greedy algorithm is correct and more efficient.

回答3:

Construct a directed graph from the array. eg: i->j if |i-j|<=x[i] (Basically, if you can move from i to j in one hop have i->j as an edge in the graph). Now, find the shortest path from first node to last.

FWIW, you can use Dijkstra's algorithm so find shortest route. Complexity is O( | E | + | V | log | V | ). Since | E | < n^2, this becomes O(n^2).

回答4:

start from left(end)..and traverse till number is same as index, use the maximum of such numbers. example if list is

   list:  2738|4|6927
   index: 0123|4|5678

once youve got this repeat above step from this number till u reach extreme right.

273846927
000001234

in case you dont find nething matching the index, use the digit with the farthest index and value greater than index. in this case 7.( because pretty soon index will be greater than the number, you can probably just count for 9 indices)

回答5:

basic idea:

start building the path from the end to the start by finding all array elements from which it is possible to make the last jump to the target element (all i such that A[i] >= target - i).

treat each such i as the new target and find a path to it (recursively).

choose the minimal length path found, append the target, return.

simple example in python:

ls1 = [2,3,1,1,4]
ls2 = [4,11,1,1,1,1,1,1,1,1,1,1,1]

# finds the shortest path in ls to the target index tgti
def find_path(ls,tgti):

    # if the target is the first element in the array, return it's index.
    if tgti<= 0:
        return [0]

    # for each 0 <= i < tgti, if it it possible to reach
    # tgti from i (ls[i] <= >= tgti-i) then find the path to i

    sub_paths = [find_path(ls,i) for i in range(tgti-1,-1,-1) if ls[i] >= tgti-i]

    # find the minimum length path in sub_paths

    min_res = sub_paths[0]
    for p in sub_paths:
        if len(p) < len(min_res):
            min_res = p

    # add current target to the chosen path
    min_res.append(tgti)
    return min_res

print  find_path(ls1,len(ls1)-1)
print  find_path(ls2,len(ls2)-1)

>>>[0, 1, 4]
>>>[0, 1, 12]

回答6:

We can calculate far index to jump maximum and in between if the any index value is larger than the far, we will update the far index value.

Simple O(n) time complexity solution

public boolean canJump(int[] nums) {
    int far = 0;
    for(int i = 0; i<nums.length; i++){
        if(i <= far){
            far = Math.max(far, i+nums[i]);
        }
        else{
            return false;
        }
    }
    return true;
}

来源：https://stackoverflow.com/questions/9041853/interview-puzzle-jump-game

标签

algorithm

pseudocode