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 ; 

    }

};

 

 

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