习题4:两个数字之间所有数的和

Example: 
get_sum(1, 0) == 1   // 1 + 0 = 1
get_sum(1, 2) == 3   // 1 + 2 = 3
get_sum(0, 1) == 1   // 0 + 1 = 1
get_sum(1, 1) == 1   // 1 Since both are same
get_sum(-1, 0) == -1 // -1 + 0 = -1
get_sum(-1, 2) == 2  // -1 + 0 + 1 + 2 = 2

def get_sum(a, b):
    min, max = sorted([a, b])
    return sum(range(min, max+1))

def get_sum(a, b):
    return sum(xrange(min(a, b), max(a, b)+1))

def get_sum(a, b):
    return (a+b)(abs(a-b)+1)//2