如何依据索引列表调整目标列表的元素顺序？

Let's break down how to solve this problem where you need to rearrange elements of a target list according to an index list that specifies where each element should go.

Understanding the Problem #

From your example:

• Target list a = ["orange", "apple", "peach", "watermelon", "gum"]
• Index list b = [3, 4, 1, 2, 0]
• The rule: For each element at original index i in a, it should be moved to position b[i] in the new list. This gives us the final result ["gum", "peach", "watermelon", "orange", "apple"].

Efficient Solution in Python #

The cleanest and most efficient way (O(n) time complexity) is to create a result list of the same length as a, then iterate through each element and place it in its designated position using the index list:

a = ["orange", "apple", "peach", "watermelon", "gum"]
b = [3, 4, 1, 2, 0]

# Initialize result list with placeholders
result = [None] * len(a)

# Iterate over original indices and their target positions
for original_idx, new_position in enumerate(b):
    result[new_position] = a[original_idx]

print(result)  # Output: ['gum', 'peach', 'watermelon', 'orange', 'apple']

Why This Works #

• enumerate(b) gives us pairs of (original_index_in_a, target_position_in_result) since each element in b corresponds to the target position of the element at the same index in a.
• We start with a placeholder list to ensure we have space for every element, then fill it in exactly where each element belongs.

Alternative (Less Efficient) Approach #

You might be tempted to use list comprehension with index(), but this has O(n²) time complexity (since index() scans the list each time), which is not ideal for large lists:

# Not recommended for large lists
result = [a[b.index(j)] for j in range(len(a))]

Stick with the first approach for better performance!

内容的提问来源于stack exchange，提问作者user3647488