https://leetcode.com/problems/find-the-duplicate-number/#/description

http://www.cnblogs.com/EdwardLiu/p/5078013.html

range search : The reason why we did not use index as "search space" for this problem is the matrix is sorted in two directions, we can not find a linear way to map the number and its index.------------(Unsorted Array)

287. Find the Duplicate Number