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].
a + b = target也可以看成是
a = target - b在map[整数]整数的序号中，可以查询到a的序号。这样就不用嵌套两个for循环了。