【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 };