Two sum（给定一个无重复数组和目标值，查找数组中和为目标值的两个数，并输出其下标）

示例：

nums = [1,2,5,7] target = [6]

return [0,2]

解决方案1：

for i in range(len(nums)):
    remain = target - nums[i]
    if remain in nums and nums.index(remain) != i:
        return [i,nums.index(remain)]

解决方案2（转载于leetcode用户Dirk41）：

data = { nums[i]: i for i,n in enumerate(nums)}
for i in range(len(nums)):
    complement = target - nums[i]
    if complement in data and data.get(complement) != i:
        return [i, data.get(complement)]

方案2的运行速度快很多倍