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描述
Given a zero-based permutation nums (0-indexed), build an array ans of the same length where ans[i] = nums[nums[i]] for each 0 <= i < nums.length and return it.
A zero-based permutation nums is an array of distinct integers from 0 to nums.length - 1 (inclusive).
Example 1:
Input: nums = [0,2,1,5,3,4]
Output: [0,1,2,4,5,3]
Explanation: The array ans is built as follows:
ans = [nums[nums[0]], nums[nums[1]], nums[nums[2]], nums[nums[3]], nums[nums[4]], nums[nums[5]]]
= [nums[0], nums[2], nums[1], nums[5], nums[3], nums[4]]
= [0,1,2,4,5,3]
Example 2:
Input: nums = [5,0,1,2,3,4]
Output: [4,5,0,1,2,3]
Explanation: The array ans is built as follows:
ans = [nums[nums[0]], nums[nums[1]], nums[nums[2]], nums[nums[3]], nums[nums[4]], nums[nums[5]]]
= [nums[5], nums[0], nums[1], nums[2], nums[3], nums[4]]
= [4,5,0,1,2,3]
Note:
1 <= nums.length <= 1000
0 <= nums[i] < nums.length
The elements in nums are distinct.
解析
根据题意，就是给出了一个整数数组 nums ，然后要将其重新进行排列，排列的规则就是在结果列表中第 i 个索引的位置上的值为 nums[nums[i]] ，所以思路比较简单，就是遍历 nums ，然后将每个 nums[num] 追加到结果 result 中，遍历结束即可得到最终的答案。
解答
class Solution(object):
def buildArray(self, nums):
"""
:type nums: List[int]
:rtype: List[int]
"""
result = []
for num in nums:
result.append(nums[num])
return result
运行结果
Runtime: 104 ms, faster than 65.36% of Python online submissions for Build Array from Permutation.
Memory Usage: 13.5 MB, less than 99.51% of Python online submissions for Build Array from Permutation.
解析
还有一种方法就是用到了 numpy 中的技巧，直接用 nums 本身作为索引，然后通过 np.array(nums)[nums] 的方式即可得到答案。按照惯例，这种方法虽然简单，但是不推荐。这里我对 numpy 中的数组技巧作一下简单的说明，代码如下：
例一：
import numpy as np
a = np.array([5,2,4,3])
print(a[[1,2]])
打印：
[2 4]
例二：
import numpy as np
a = np.array([5,2,4,3])
print(a[[0,1]])
打印：
[5 2]
先将索引列表传入入 numpy 所构造的数组中，即可得到新的数组。
从结果来看，这种方法的速度也是很慢的，而且占用的内存较大。
解答
class Solution(object):
def buildArray(self, nums):
"""
:type nums: List[int]
:rtype: List[int]
"""
import numpy as np
return np.array(nums)[nums]
运行结果
Runtime: 144 ms, faster than 19.28% of Python online submissions for Build Array from Permutation.
Memory Usage: 26 MB, less than 9.80% of Python online submissions for Build Array from Permutation.
原题链接：leetcode.com/problems/bu…
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