【leetcode】First Missing Positive

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.

 

通过swap操作，把各个元素swap到相应的位置上，然后再扫描一遍数组，确定丢失的正数的位置

 

 1 class Solution {

 2 public:

 3     int firstMissingPositive(int A[], int n) {

 4        

 5         if(n==0) return 1;

 6        

 7         for(int i=0;i<n;i++)

 8         {

 9             if(A[i]!=i+1&&A[i]<=n&&A[i]>=1&&A[i]!=A[A[i]-1])

10             {

11                 swap(A[i],A[A[i]-1]);

12                 i--;

13             }

14         }

15        

16         for(int i=0;i<n;i++)

17         {

18             if(A[i]!=i+1) return i+1;

19  

20         }

21         return n+1;

22        

23     }

24    

25     void swap(int &a,int &b)

26     {

27         int tmp=a;

28         a=b;

29         b=tmp;

30     }

31 };