1. Two Sum

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,
and you may not use the same element twice.

Example:

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

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

First

目標和為target、num為遍歷數(shù)組每個位置的值，index為數(shù)組的索引
將遍歷過的數(shù)字所對應(yīng)的index緩存在字典中，如果目標target-num出現(xiàn)在字典中。
則將字典中的索引和當前值的索引組成答案返回。

代碼時間復(fù)雜度為O(n)

class Solution(object):
    def twoSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        """
        location = {}
        for index, num in enumerate(nums):
            if target - num in location:
                return [location[target-num], index]
            location[num] = index
        return [-1, -1]

Fastest