LeetCode_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.

技巧性比较强：
参考： http://blog.unieagle.net/2012/09/20/leetcode%E9%A2%98%E7%9B%AE%EF%BC%9Afirst-missing-positive/

class Solution {
public:
    int firstMissingPositive(int A[], int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
       int outOfRange = n + 2;
        for(int i = 0; i< n; i++)
           if(A[i]<=0)
              A[i] = outOfRange ;
        int absi;
        for(int i = 0; i< n; i++)
        {
            absi = abs(A[i]);
            if(absi <= n)
            {
                A[absi-1] = -abs(A[absi-1]) ;
            }
        }
        
        for(int i = 0; i< n; i++)
           if(A[i] >0) 
              return i+1;
      return n+1 ; 
    }
};