First Missing Positive


Given an unsorted integer array, find the first missing positive integer.
For example,
Given [1,2,0] return 3,
and [3,4,-1,1] return 2.
Your algorithm should run in O(n) time and uses constant space.
Solution: hash-table solution (intuitive approach, O(n) in time and space...)

The following code satisfies the constant space requirement. The key idea is: the first missed positive integer must in [1, n+1]. If we swap the integers and make sure and A[i] = i, we can easily check that which integer is missed. 
Revised it at 2013-09-23:

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