45. Jump Game II

#Problem

You are given a 0-indexed array of integers nums of length n. You are initially positioned at nums[0].

Each element nums[i] represents the maximum length of a forward jump from index i. In other words, if you are at nums[i], you can jump to any nums[i + j] where:

• 0 <= j <= nums[i] and
• i + j < n

Return the minimum number of jumps to reach nums[n - 1]. The test cases are generated such that you can reach nums[n - 1].

https://leetcode.cn/problems/jump-game-ii/

Example 1:

Input: nums = [2,3,1,1,4]
Output: 2
Explanation: The minimum number of jumps to reach the last index is 2. Jump 1 step from index 0 to 1, then 3 steps to the last index.

Example 2:

Input: nums = [2,3,0,1,4]
Output: 2

Constraints:

• 1 <= nums.length <= 10⁴
• 0 <= nums[i] <= 1000
• It’s guaranteed that you can reach nums[n - 1].

#Test Cases

class Solution:
    def jump(self, nums: List[int]) -> int:

import pytest

from solution import Solution


@pytest.mark.parametrize('nums, expected', [
    ([2,3,1,1,4], 2),
    ([2,3,0,1,4], 2),
])
@pytest.mark.parametrize('sol', [Solution()])
def test_solution(sol, nums, expected):
    assert sol.jump(nums) == expected

#Thoughts

跟 55. Jump Game 类似。

记 dp(i) 表示从位置 i 跳到 n - 1 的最少跳数，可知

即从位置 i 先跳一次到 j，再从 j 用最少次数跳到 n - 1。所有可能的跳法中取总次数最少的。

初值 dp(n - 1) = 0，结果取 dp(0)。

时间复杂度 O(n * m)，空间复杂度 O(n)，其中 m 是 nums 中数值的大小。

#Code

class Solution:
    def jump(self, nums: list[int]) -> int:
        n = len(nums)
        dp = [0] * n
        for i in range(n - 2, -1, -1):
            dp[i] = 1 + min(dp[i + 1:i + nums[i] + 1], default=n)

        return dp[0]

#Faster

O(n) 时间的算法，TODO。