You have k lists of sorted integers. Find the smallest range that includes at least one number from each of the k lists. 

For example, 
List 1: [4, 10, 15, 24, 26] 
List 2: [0, 9, 12, 20] 
List 3: [5, 18, 22, 30] 

The smallest range here would be [20, 24] as it contains 24 from list 1, 20 from list 2, and 22 from list 3.

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我的思路是先取最小的triple，然后最小的那个，增加。迭代。

一道题目- Find the smallest range that includes at least one number from each of the k lists