[Google]find a,b,c such that a+b+c

Question:

Suppose you have a million integer numbers. 

Return all possible values of a,b and c such that 
a+b+c<=d. 
d will be provided to you. 
ex: if the numbers are 1,2,3,4,5,6,7 
and d=7 
[1,2,3] 
[1,2,4] 

[1,2,3] will be same as [1,3,2] and [3,2,1]... 

follow up: 

Return all such parts that satisfy the above condition if d is not provided.

Solution:

k-sum的問題一般有兩種解法.

第一種:1)排序2)固定k-2個數位置3)兩個指針分別從前往後和從後往前移動.

第二種:緩存k/2的數的和(和爲key,數字爲值),然後對剩餘k/2個數遍歷.

這題說到million,不知道有沒有額外的技巧.望高人回答.