【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==0) return 1;
       
        for(int i=0;i<n;i++)
        {
            if(A[i]!=i+1&&A[i]<=n&&A[i]>=1&&A[i]!=A[A[i]-1])
            {
                swap(A[i],A[A[i]-1]);
                i--;
            }
        }
       
        for(int i=0;i<n;i++)
        {
            if(A[i]!=i+1) return i+1;
 
        }
        return n+1;
       
    }
   
    void swap(int &a,int &b)
    {
        int tmp=a;
        a=b;
        b=tmp;
    }
};