LeetCode #1: Two Sum

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本文介绍了一种解决'两数之和'问题的有效算法。该算法通过使用字典存储已遍历过的数值及其索引，实现了快速查找目标补数的过程。文章详细解释了算法的实现过程，并给出了具体示例，最后分析了算法的时间复杂度为O(n)，空间复杂度为O(1)。

Problem Statement

(Problem Link) Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution.

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

Solution

class Solution(object):
    def twoSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        """
        indexed_map = dict()
        for i, v in enumerate(nums):
            if target - v in indexed_map:
                return [indexed_map[target - v], i]
            else:
                indexed_map[v] = i

Time complexity: $O(n)$
Space complexity: $O(1)$