【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

 

 class Solution {
 public:
     int firstMissingPositive(int A[], int n) {

         if(n==) return ;

         for(int i=;i<n;i++)
         {
             if(A[i]!=i+&&A[i]<=n&&A[i]>=&&A[i]!=A[A[i]-])
             {
                 swap(A[i],A[A[i]-]);
                 i--;
             }
         }

         for(int i=;i<n;i++)
         {
             if(A[i]!=i+) return i+;

         }
         return n+;

     }

     void swap(int &a,int &b)
     {
         int tmp=a;
         a=b;
         b=tmp;
     }
 };