http://www.cnblogs.com/grandyang/p/8440087.html
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].
这个代码时间复杂度是o(n^2),
a = [2,7,11,15]
b = 9
c = len(a)
for i in range(0,c):
for j in range(0,c):
if i != j and a[i] + a[j] ==b:
print(i,j)
减小到一次0(n)